Calcium

What it is

Calcium in the medicine cabinet is a salt, not pure calcium, and the two numbers on the pack are not the same thing. One TUMS Ultra chewable tablet contains 1000 mg of calcium carbonate, which the label says provides 410 mg of elemental calcium. The same mineral is sold in three regulatory guises: as an over-the-counter antacid, where it neutralises stomach acid; as a prescription phosphate binder (calcium acetate) for people on dialysis; and as a food supplement. The Institute of Medicine set reference intakes for it in 2011 based on bone health.

What the research says

As an antacid it works, briefly: crossover trials show chewable calcium carbonate raises oesophageal and gastric pH and cuts heartburn, but the effect on gastric acidity is short-lasting. As a phosphate binder in dialysis it lowers phosphate, but Cochrane found sevelamer may halve all-cause death and cause far less high blood calcium than calcium-based binders. As a bone supplement the picture is disappointing: a JAMA meta-analysis of 33 trials and 51,145 people found no reduction in hip or any other fracture, and the 36,282-woman WHI trial found no significant reduction in hip fracture but a 17% relative increase in kidney stones. Calcium does lower blood pressure very slightly in people with normal pressure (about 1.4 mmHg systolic, high certainty). Whether calcium supplements raise heart attack risk is genuinely disputed, and over-use of calcium antacids can cause milk-alkali syndrome.

Evidence grade: Disputed.

How it works

Drug class: Mineral salt used as three different medicines: an over-the-counter antacid (calcium carbonate), a prescription phosphate binder (calcium acetate), and a nutritional supplement

Which job the calcium does depends on where it is and what it is paired with. In the stomach the carbonate salt neutralises acid directly. In the gut, taken with meals, calcium binds dietary phosphate into an insoluble complex that leaves in the stool, which is how calcium acetate lowers blood phosphate in kidney failure. The same binding habit is why calcium also grabs onto other medicines and stops them being absorbed. Separately, calcium is the main mineral in bone, which is the basis for the reference intakes. (Source 1)

What it is used for

• Chewable calcium carbonate raises oesophageal pH and reduces heartburn after a meal compared with placebo in crossover trials, but the effect on stomach acidity is short-lived and an antacid chewing gum outlasted the tablets. Evidence: limited. (Source 2)
• Calcium acetate does bind phosphate, but Cochrane’s 2025 review of 134 trials found that compared with calcium, sevelamer may reduce death from any cause and causes far less high blood calcium in people on dialysis, all on low-certainty evidence. Evidence: disputed. (Source 3)
• A 2017 JAMA meta-analysis of 33 trials in 51,145 community-dwelling adults over 50 found no reduction in hip, non-vertebral, vertebral or total fracture. The WHI trial of 36,282 postmenopausal women found hip bone density 1.06% higher but a hip-fracture hazard ratio of 0.88 (0.72 to 1.08), not significant, and more kidney stones. Evidence: not-supported. (Source 4)
• The 2025 Cochrane update, which excluded 20 previously included trials over eligibility and trustworthiness, found calcium may make little to no difference to pre-eclampsia (RR 0.83, 0.67 to 1.04), and the sensitivity analysis restricted to large trials gave high-certainty evidence of little to no difference. Evidence: not-supported. (Source 5)
• Cochrane 2022 found high-certainty evidence that extra calcium lowers systolic pressure by 1.37 mmHg and diastolic by 1.45 mmHg in normotensive people, a real but very small effect. None of the 20 included studies reported adverse events. Evidence: limited. (Source 6)
• Two randomised trials in 1,346 people with previous adenomas found fewer recurrent adenomas with calcium (OR 0.74, 0.58 to 0.95), but Cochrane concluded this is not enough evidence to recommend calcium supplements to prevent colorectal cancer. Evidence: limited. (Source 7)

Interactions

• Levothyroxine (thyroid hormone) (pharmacokinetic study): Calcium carbonate, citrate and acetate all cut levothyroxine absorption by about a fifth to a quarter, which can under-treat an underactive thyroid. (Source 8)
• Ciprofloxacin and other fluoroquinolone antibiotics (pharmacokinetic study): Calcium binds the antibiotic in the gut. Regular calcium carbonate cut ciprofloxacin peak levels by 40% and total exposure by 43%, enough to matter in a serious infection. Limit: This paper has no DOI. The DOI previously recorded for it belongs to a different article in the same journal and month, so it has been removed; the paper is identified by PMID 8471407 and PMC1381579, and the quoted abstract was located in that record. It is a two-period crossover study in six healthy men, so it measures blood levels of the antibiotic, not whether infections were treated less successfully. (Source 9)
• Tetracycline antibiotics (label): Calcium binds drugs with carboxyl and hydroxyl groups, which the prescription calcium label says can reduce the availability of tetracyclines as well as fluoroquinolones. (Source 1)
• Oral medicines generally, where timing matters (label): Where reduced absorption would matter, the prescription calcium label says to give the other drug one hour before or three hours after the calcium, and to monitor levels of narrow-margin drugs. (Source 1)
• Other calcium supplements and calcium-based antacids taken alongside a prescription calcium binder (label): Stacking an over-the-counter calcium antacid on top of prescription calcium acetate is what tips dialysis patients into high blood calcium; the label says to avoid it. (Source 10)
• Dicyclomine and other anticholinergic antispasmodics (label): This one runs the other way: the dicyclomine label says antacids may interfere with the absorption of anticholinergic drugs including dicyclomine, so the two should not be taken together. The calcium antacid's own label names no drug, only prescription drugs in general. (Source 11)
• Vitamin D (clinical trial): Vitamin D increases calcium absorption, and the two are set together: the Institute of Medicine derived each nutrient’s reference intake assuming requirements for the other were met. In the WHI trial the two were given together, and the combination raised kidney stone risk. (Source 12)
• Food (meals) (label): Calcium acetate only binds phosphate when it is taken with food, which is why the dosing is per meal rather than per day. (Source 13)
• Alcohol (label): No interaction between calcium salts and alcohol is documented in any source we reached. The over-the-counter antacid label names no alcohol warning; its only interaction warning is the general one about prescription drugs. (Source 14)

Stopping it

• For the over-the-counter antacid the label sets its own ceiling and its own stopping point: no more than 7 tablets in 24 hours, no more than 5 if pregnant, and not at the maximum dose for more than 2 weeks except under a doctor's advice and supervision. (Source 14)
• The same label's directions give the other stopping point: symptoms that go on for more than two weeks should not keep being treated with it without a doctor's advice. (Source 15)
• Stopping is the treatment when calcium causes trouble. In the milk-alkali case, the man’s serum calcium normalised within six days of stopping the antacid, with fluids and one dose of a bisphosphonate, and his kidney function was still improving 18 months later. (Source 16)
• For prescription calcium acetate, high blood calcium is handled by cutting the dose or stopping the drug, with acute haemodialysis for severe cases; the label also says to reduce or stop vitamin D. No withdrawal syndrome is described for any calcium product in the sources we reached. (Source 10)

What goes wrong

A patient-level and trial-level meta-analysis found calcium supplements without vitamin D were associated with a higher rate of myocardial infarction. (Source 17)

• Meta-analysis, Low certainty.
• Size: 15 trials: 8,151 participants with patient-level data and 11,921 with trial-level data.
• Who: participants of mean age over 40 in randomised placebo-controlled trials of calcium supplements of at least 500 mg a day.
• How long: median follow-up 3.6 years (patient-level); mean 4.0 years (trial-level)
• Result: Patient-level: 143 myocardial infarctions on calcium versus 111 on placebo, hazard ratio 1.31 (95% CI 1.02 to 1.67, P=0.035). Stroke 1.20 (0.96 to 1.50, P=0.11); composite of myocardial infarction, stroke or sudden death 1.18 (1.00 to 1.39, P=0.057); death 1.09 (0.96 to 1.23, P=0.18). Trial-level: pooled relative risk 1.27 (1.01 to 1.59, P=0.038).
• Funding: independent.

In the five studies contributing patient level data, 143 people allocated to calcium had a myocardial infarction compared with 111 allocated to placebo (hazard ratio 1.31, 95% confidence interval 1.02 to 1.67, P=0.035). Non-significant increases occurred in the incidence of stroke (1.20, 0.96 to 1.50, P=0.11), the composite end point of myocardial infarction, stroke, or sudden death (1.18, 1.00 to 1.39, P=0.057), and death (1.09, 0.96 to 1.23, P=0.18).

In people on dialysis, Cochrane found sevelamer may reduce death from any cause and may cause much less high blood calcium than calcium-based phosphate binders, on low-certainty evidence, with little or no difference in serum phosphate. (Source 18)

• Systematic review, Low certainty.
• Size: 19 studies, 4,403 participants for death; 20 studies, 4,124 participants for hypercalcaemia (134 studies, 20,913 adults in the review)
• Who: adults with chronic kidney disease, mostly on dialysis for the head-to-head comparisons.
• How long: median study duration 5.4 months.
• Result: Compared with calcium, sevelamer: death from any cause RR 0.54 (95% CI 0.32 to 0.93); hypercalcaemia RR 0.30 (95% CI 0.20 to 0.43); nausea RR 0.98 (0.56 to 1.71); constipation RR 1.35 (0.71 to 2.57); serum phosphate mean difference 0.08 mg/dL (-0.09 to 0.24). All low certainty.
• Funding: independent.

Limit of this finding: Every estimate in this comparison is graded low certainty by the review itself, which means the true effect could differ substantially. The mortality result (RR 0.54, 0.32 to 0.93) comes from 19 studies and its interval comes close to 1, so it should not be read as a settled halving of risk.

Compared to calcium, sevelamer may reduce death from any cause (RR 0.54, 95% CI 0.32 to 0.93; 19 studies, 4403 participants; low‐certainty evidence), have uncertain effects on cardiovascular death, may result in less hypercalcaemia (RR 0.30, 95% CI 0.20 to 0.43; 20 studies, 4124 participants; low‐certainty evidence)

Overusing calcium carbonate antacids can cause milk-alkali syndrome: high blood calcium, metabolic alkalosis and acute kidney injury. (Source 16)

• Case report, Very low certainty.
• Size: 1 patient.
• Who: a 42-year-old man who chronically overused calcium carbonate tablets for reflux.
• How long: four weeks of symptoms before presentation; calcium normalised within six days of stopping.
• Result: Hypercalcaemia, metabolic alkalosis and acute kidney injury, treated with intravenous fluids, stopping the antacid and a single 4 mg dose of zoledronic acid; creatinine was still improving at 18 months.
• Funding: not stated.

Diagnostic evaluation revealed hypercalcaemia, metabolic alkalosis, and acute kidney injury, necessitating intravenous fluid administration and cessation of calcium carbonate tablet consumption. Comprehensive investigations excluded alternative aetiologies, confirming Milk-Alkali Syndrome (MAS).

On prescription calcium acetate the label reports high blood calcium above 11 mg/dL in 16% of patients in a three-month study, tells prescribers to check calcium twice weekly while the dose is being set, and says chronic high calcium can cause calcification in blood vessels and other soft tissue. (Source 10)

• Official position, Certainty not rated.
• Size: not stated.
• Who: patients with end stage renal disease taking calcium acetate.
• How long: ongoing treatment.
• Result: Hypercalcaemia above 11 mg/dL was reported in 16% of patients in a 3-month study of the solid dose formulation, and all cases resolved on lowering the dose or stopping. The label defines mild hypercalcaemia as 10.5 to 11.9 mg/dL and severe as above 12 mg/dL, treatable with acute haemodialysis and stopping the drug.
• Funding: not stated.

Limit of this finding: The 16% is from a three-month study of the solid dose formulation of prescription calcium acetate, taken as a phosphate binder. The quoted sentence names only that study; the population comes from the opening sentence of the same section of the label, 'Patients with end stage renal disease may develop hypercalcemia when treated with calcium, including calcium acetate.' So this is a rate in dialysis patients on a prescribed binder, not a rate for calcium supplements or antacids in people with normal kidneys. The label gives no denominator and no study identifier for it.

Hypercalcemia (>11 mg/dL) was reported in 16% of patients in a 3-month study of solid dose formulation of calcium acetate; all cases resolved upon lowering the dose or discontinuing treatment.

Calcium carbonate, calcium citrate and calcium acetate each cut levothyroxine absorption by about a fifth to a quarter in a single-dose pharmacokinetic study. (Source 8)

• Blood level study, Low certainty.
• Size: 8 healthy euthyroid adults.
• Who: healthy euthyroid adults in single-dose crossover pharmacokinetic studies.
• How long: serum thyroxine measured over 6 hours after each dose.
• Result: All three calcium preparations, each containing 500 mg elemental calcium, reduced levothyroxine absorption by about 20% to 25% compared with levothyroxine alone.
• Funding: not stated.

Coadministration of each of the three calcium preparations significantly reduced levothyroxine absorption by about 20%–25% compared with levothyroxine given alone. Conclusions Contrary to a prior report, our data suggest that calcium acetate interferes with levothyroxine absorption in a manner similar to that seen with calcium carbonate and calcium citrate.

Taking calcium carbonate regularly cut the peak blood level and total exposure of oral ciprofloxacin by roughly 40%. (Source 9)

• Blood level study, Low certainty.
• Size: 6 healthy male volunteers.
• Who: healthy male volunteers in a two-period, two-treatment crossover study.
• How long: chronic calcium carbonate dosing with a single ciprofloxacin dose.
• Result: Mean reduction of 40% in Cmax and 43% in AUC with calcium carbonate (P < 0.05), with no change in half-life or time to peak.
• Funding: not stated.

Limit of this finding: This paper has no DOI. The DOI previously recorded for it belongs to a different article in the same journal and month, so it has been removed; the paper is identified by PMID 8471407 and PMC1381579, and the quoted abstract was located in that record. It is a two-period crossover study in six healthy men, so it measures blood levels of the antibiotic, not whether infections were treated less successfully.

There was a mean reduction of 40% in Cmax and 43% in AUC when calcium carbonate was administered with ciprofloxacin, compared with ciprofloxacin alone (P < 0.05). There were no changes in either half-life or tmax.

What the evidence supports

Cochrane found that extra calcium lowers blood pressure in people with normal blood pressure, but by about 1.4 mmHg systolic and 1.5 mmHg diastolic. (Source 6)

• Systematic review, High certainty.
• Size: 20 trials with 3,512 participants, 18 trials with 3,140 participants in the meta-analysis.
• Who: normotensive people randomised to dietary calcium supplementation or fortification versus placebo or control.
• How long: varied; searched to September 2020.
• Result: Systolic mean difference -1.37 mmHg (95% CI -2.08 to -0.66; 3,140 participants; 18 studies; I2 = 0%; high certainty). Diastolic mean difference -1.45 (95% CI -2.23 to -0.67; 3,039 participants; 17 studies; I2 = 45%; high certainty). None of the studies reported hypertension as a dichotomous outcome.
• Funding: independent.

Limit of this finding: The review's own certainty labels are not internally consistent here: it grades the diastolic result high certainty while reporting substantial disagreement between the trials (I2 = 45%), and elsewhere in the same results it grades an I2 = 51% result high certainty and an I2 = 0% result only moderate. Treat the certainty labels with caution; the sizes of the effects, about 1.4 to 1.5 mmHg, are what the trials measured.

None of the studies reported hypertension as a dichotomous outcome. The effect on systolic and diastolic blood pressure was: mean difference (MD) ‐1.37 mmHg, 95% confidence interval (CI) ‐2.08, ‐0.66; 3140 participants; 18 studies; I2 = 0%, high‐certainty evidence; and MD ‐1.45, 95% CI ‐2.23, ‐0.67; 3039 participants; 17 studies; I2 = 45%, high‐certainty evidence, respectively.

Chewable calcium carbonate antacid tablets raised oesophageal pH and reduced heartburn after a provocative meal, though an antacid chewing gum worked for longer. (Source 2)

• Randomized trial, Low certainty.
• Size: 24 subjects in a four-way crossover.
• Who: subjects with heartburn, given a standardised provocative meal with dual pH probe monitoring.
• How long: symptoms recorded every 15 minutes for 120 minutes after dosing.
• Result: Regimens were placebo, chewable antacid tablets containing 1000 mg calcium carbonate, lower-dose gum (600 mg) and higher-dose gum (900 mg). Active chewable antacid and both gums immediately raised oesophageal pH with significant improvement 15 to 30 minutes after dosing; the higher-dose gum beat the tablets for up to 120 minutes.
• Funding: not stated (a product development study of a new antacid gum)

Higher dose gum decreased heartburn more than chewable antacids up to 120 min post-dose. pH: Active chewable antacid and gums immediately increased oesophageal pH, with significant improvement 15-30 min post-dose.

What the evidence does not support

A meta-analysis of 33 randomised trials in over 51,000 community-dwelling older adults found calcium, vitamin D or both did not reduce hip or any other fracture. (Source 4)

• Meta-analysis, Moderate certainty.
• Size: 33 randomised trials, 51,145 participants.
• Who: community-dwelling adults older than 50 years.
• How long: varied by trial; trials searched to 2017.
• Result: Hip fracture: calcium RR 1.53 (95% CI 0.97 to 2.42), absolute risk difference 0.01 (0.00 to 0.01); vitamin D RR 1.21 (0.99 to 1.47); combined calcium and vitamin D RR 1.09 (0.85 to 1.39), absolute risk difference 0.00 (-0.00 to 0.00). No significant associations for non-vertebral, vertebral or total fracture.
• Funding: not stated in the abstract.

Limit of this finding: Two things to read carefully in this abstract. The hip-fracture point estimates for calcium (1.53) and vitamin D (1.21) sit above 1, but neither confidence interval excludes 1, so the trials showed no reduction and no proven increase either. And the published abstract leaves a bracket unclosed in the vitamin D figure; that is the journal's typographical slip, not a missing number.

There was no significant association of calcium or vitamin D with risk of hip fracture compared with placebo or no treatment (calcium: RR, 1.53 [95% CI, 0.97 to 2.42]; ARD, 0.01 [95% CI, 0.00 to 0.01]; vitamin D: RR, 1.21 [95% CI, 0.99 to 1.47]; ARD, 0.00 [95% CI, -0.00 to 0.01]. There was no significant association of combined calcium and vitamin D with hip fracture compared with placebo or no treatment (RR, 1.09 [95% CI, 0.85 to 1.39]; ARD, 0.00 [95% CI, -0.00 to 0.00]).

The authors of that meta-analysis concluded the findings do not support routine calcium or vitamin D supplements in community-dwelling older people. (Source 19)

• Meta-analysis, Moderate certainty.
• Size: 33 randomised trials, 51,145 participants.
• Who: community-dwelling adults older than 50.
• How long: varied.
• Result: See the pooled estimates above; this finding records the authors’ own conclusion.
• Funding: not stated in the abstract.

the use of supplements that included calcium, vitamin D, or both compared with placebo or no treatment was not associated with a lower risk of fractures among community-dwelling older adults. These findings do not support the routine use of these supplements in community-dwelling older people.

A 2016 systematic review for the National Osteoporosis Foundation reached the opposite conclusion: trials showed no significant difference in cardiovascular events or mortality with calcium supplements. (Source 20)

• Systematic review, Moderate certainty.
• Size: 4 randomised trials in 10 publications, plus 27 observational studies.
• Who: generally healthy adults.
• How long: varied; literature searched to July 2016.
• Result: No pooled risk estimate is given in the abstract. The review reports no statistically significant differences in cardiovascular events or mortality, and inconsistent dose-response relationships in cohort studies. Its stated conclusion is that intake within tolerable upper intake levels of 2000 to 2500 mg a day is not associated with cardiovascular risk.
• Funding: industry-adjacent: primary funding source National Osteoporosis Foundation.

The trials did not find statistically significant differences in risk for CVD events or mortality between groups receiving supplements of calcium or calcium plus vitamin D and those receiving placebo. Cohort studies showed no consistent dose-response relationships between total, dietary, or supplemental calcium intake levels and cardiovascular mortality and highly inconsistent dose-response relationships between calcium intake and risks for total stroke or stroke mortality.

The 2025 Cochrane update found calcium in pregnancy may make little to no difference to pre-eclampsia, with high-certainty evidence of no difference once only the large trials are counted. (Source 5)

• Systematic review, High certainty.
• Size: 6 RCTs, 15,364 women for the main pre-eclampsia analysis (10 RCTs, 37,504 participants in the review)
• Who: pregnant women randomised to calcium supplementation versus placebo, all receiving standard care.
• How long: pregnancy.
• Result: Pre-eclampsia RR 0.83 (95% CI 0.67 to 1.04), risk difference 9 per 1000 fewer (17 fewer to 2 more), low certainty. Sensitivity analysis excluding studies under 500 participants: RR 0.92 (0.79 to 1.05), 4 RCTs, 14,730 women, high certainty. Maternal death RR 0.33 (0.06 to 1.83), very low certainty. Adverse effects RR 2.16 (0.43 to 10.78), very low certainty.
• Funding: independent.

Sensitivity analysis excluding small studies (fewer than 500 participants) indicates little to no difference in pre‐eclampsia (RR 0.92, 95% CI 0.79 to 1.05; 4 RCTs, 14,730 women; high‐certainty evidence). The evidence is very uncertain about the effect of calcium on maternal death (RR 0.33, 95% CI 0.06 to 1.83; 3 RCTs, 9430 women; very low‐certainty evidence) and on adverse effects (RR 2.16, 95% CI 0.43 to 10.78; 2 RCTs, 714 women; very low‐certainty evidence).

The Institute of Medicine committee that set the 2011 reference intakes judged the evidence for calcium outside bone health - cancer, cardiovascular disease, diabetes, autoimmune disease - inconsistent and insufficient. (Source 21)

• Official position, Moderate certainty.
• Size: a comprehensive evidence review by a 14-scientist committee.
• Who: normal healthy persons in the North American population.
• How long: not applicable.
• Result: Reference Dietary Allowances for calcium of 700 to 1300 mg a day for life-stage groups at least 1 year old, based on bone health alone.
• Funding: independent (commissioned by US and Canadian government agencies)

The evidence, however, is not yet compelling that either nutrient confers benefits for, or is causally related to, extraskeletal health outcomes. Moreover, existing evidence suggests that nearly all individuals meet their needs at intake levels (RDAs) provided in this report and, for vitamin D, at 25OHD levels of at least 20 ng/ml (50 nmol/liter) even under conditions of minimal sun exposure. Furthermore, higher levels have not been shown consistently to confer greater benefits, challenging the concept that “more is better.”

Where the evidence is mixed

In the largest single trial, 1000 mg of elemental calcium as calcium carbonate with 400 IU of vitamin D daily raised hip bone density slightly, did not significantly reduce hip fracture, and increased kidney stones. (Source 12)

• Randomized trial, High certainty.
• Size: 36,282 postmenopausal women.
• Who: postmenopausal women aged 50 to 79 already enrolled in a Women’s Health Initiative clinical trial.
• How long: average follow-up 7.0 years.
• Result: Hip bone density 1.06% higher than placebo (P<0.01). Hazard ratios: hip fracture 0.88 (95% CI 0.72 to 1.08), clinical spine fracture 0.90 (0.74 to 1.10), total fractures 0.96 (0.91 to 1.02). Renal calculi hazard ratio 1.17 (95% CI 1.02 to 1.34). Censoring non-adherent women reduced the hip fracture hazard ratio to 0.71 (0.52 to 0.97).
• Funding: independent (US National Institutes of Health programme)

Intention-to-treat analysis indicated that participants receiving calcium plus vitamin D supplementation had a hazard ratio of 0.88 for hip fracture (95 percent confidence interval, 0.72 to 1.08), 0.90 for clinical spine fracture (0.74 to 1.10), and 0.96 for total fractures (0.91 to 1.02). The risk of renal calculi increased with calcium plus vitamin D (hazard ratio, 1.17; 95 percent confidence interval, 1.02 to 1.34).

That blood-pressure review cannot speak to safety: none of its included trials reported adverse events at all, which is not the same as none occurring. (Source 22)

• Systematic review, High certainty.
• Size: 20 trials, 3,512 participants.
• Who: normotensive people.
• How long: varied.
• Result: Not applicable - adverse events were not reported by any included study.
• Funding: independent.

Limit of this finding: "None of the studies reported adverse events" means no included trial collected or published adverse-event data. It is not evidence that extra calcium caused no harm. Note also that 20 trials were included in the review but only 18 contributed to the pooled blood-pressure estimates.

None of the studies reported adverse events. Authors' conclusions An increase in calcium intake slightly reduces both systolic and diastolic blood pressure in normotensive people, particularly in young people, suggesting a role in the prevention of hypertension. The effect across multiple prespecified subgroups and a possible dose response effect reinforce this conclusion. Even small reductions in blood pressure could have important health implications for reducing vascular disease. A 2 mmHg lower systolic blood pressure is predicted to produce about 10% lower stroke mortality and about 7% lower mortality from ischaemic heart disease. There is a great need for adequately‐powered clinical trials randomising young people. Subgroup analysis should involve basal calcium intake, age, sex, basal blood pressure, and body mass index. We also require assessment of side effects, optimal doses and the best strategy to improve calcium intake.

The same Cochrane update threw out twenty trials it had previously included, nine of them because of trustworthiness problems, which is why the earlier favourable result shrank. (Source 23)

• Systematic review, Moderate certainty.
• Size: 10 RCTs with 37,504 participants retained; 20 previously included trials excluded.
• Who: pregnant women.
• How long: pregnancy.
• Result: Not applicable - a methodological change, not an effect estimate.
• Funding: independent.

We excluded 20 previously included trials; 11 because eligibility criteria changed and nine because they are awaiting classification due to trustworthiness issues.

Two randomised trials found calcium supplements reduced recurrence of colorectal adenomas, but Cochrane judged this insufficient to recommend calcium for preventing colorectal cancer. (Source 7)

• Systematic review, Low certainty.
• Size: 2 studies, 1,346 subjects.
• Who: adults with previous adenomas.
• How long: mean 4 years in one trial and 3 years in the other.
• Result: Recurrent colorectal adenoma odds ratio 0.74 (95% CI 0.58 to 0.95). Doses were 1200 mg a day of elemental calcium for a mean of 4 years, and 2000 mg a day for three years. Loss to follow-up 14% and 11%.
• Funding: independent.

Limit of this finding: Both trials enrolled people who had already had a colorectal adenoma, so the odds ratio of 0.74 is about preventing a further adenoma in that group, not about preventing bowel cancer in the general population. Cochrane's own conclusion is that this is not enough to recommend calcium for preventing colorectal cancer.

For the development of recurrent colorectal adenoma, a reduction was found (OR 0.74, CI 0.58,0.95) when the results from both trials were combined. Authors' conclusions Although the evidence from two RCTs suggests that calcium supplementation might contribute to a moderate degree to the prevention of colorectal adenomatous polyps, this does not constitute sufficient evidence to recommend the general use of calcium supplements to prevent colorectal cancer.

In a 24-person crossover study, a chewable antacid containing calcium carbonate plus magnesium carbonate raised stomach pH compared with no drug for about the first half hour, as did the magaldrate gel it was tested against. (Source 24)

• Randomized trial, Low certainty.
• Size: 24 healthy adult volunteers (11 males, 13 females)
• Who: fasting healthy volunteers in an open, randomised, placebo-controlled two-centre crossover study.
• How long: intragastric pH measured for 3 hours.
• Result: Both antacids differed significantly from placebo in median pH during the first 30 minutes (p < 0.05), but only the magaldrate gel raised pH significantly in the first 5 minutes. The calcium-containing product supplied 680 mg calcium carbonate plus 80 mg magnesium carbonate in 2 tablets.
• Funding: not stated.

Limit of this finding: Three limits on this result. The product tested was not calcium alone: two tablets supplied 680 mg calcium carbonate plus 80 mg magnesium carbonate, against a magaldrate 800 mg gel. The 30-minute pH difference applies to both antacids, not to the calcium one on its own, and only the magaldrate gel raised pH significantly in the first five minutes. And the "placebo" arm was no drug at all, a control rather than a dummy tablet. The abstract also names its primary endpoint - the median time before intragastric pH stayed above 3.0 for ten consecutive minutes - and never reports it, so the 30-minute figure the claim rests on is a secondary observation.

For both antacids, the median pH during the first 30 min after drug administration was statistically significantly different from placebo (p < 0.05), but there was a statistically significant increase in pH during the first 5 min for Riopan Gel only. CONCLUSION: Compared to placebo, both antacids (Rennie and Riopan Gel) have short-lasting effects on intragastric acidity.

The study behind calcium acetate's approval was open-label and not randomised, with no control group, and the label itself says the true size of the phosphate-lowering effect is therefore uncertain. (Source 25)

• Case series, Very low certainty.
• Size: 91 patients with end-stage renal disease on haemodialysis.
• Who: hyperphosphataemic haemodialysis patients (serum phosphorus above 5.5 mg/dL) after a 1-week phosphate binder washout.
• How long: 12 weeks.
• Result: Starting dose 2 tablets of 667 mg per meal three times a day; average final dose 3.4 tablets per meal. Phosphorus fell from 7.4 to 5.2 mg/dL, a 30% decrease over 12 weeks (p<0.01), while serum calcium rose 9%.
• Funding: industry-funded (manufacturer trials submitted for approval)

Limit of this finding: This was an open-label, non-randomised, single-arm study of 91 patients, so it is recorded here as a case series rather than a randomised trial: the schema has no value for an uncontrolled interventional study. Everyone received the drug, so the fall in phosphorus cannot be separated from the effect of the diet, the washout or the passage of time. The label says this itself.

Although there was a decrease in serum phosphorus, in the absence of a control group the true magnitude of effect is uncertain.

In the placebo-controlled crossover arm, two weeks of calcium acetate lowered serum phosphorus by a mean of 19% and raised serum calcium by 7%, a rise the label itself describes as statistically significant but clinically unimportant. (Source 3)

• Randomized trial, Moderate certainty.
• Size: patients from the open-label study whose serum phosphorus exceeded 5.5 mg/dL after stopping their binder.
• Who: haemodialysis patients with end-stage renal disease.
• How long: 2 weeks per treatment arm in a double-blind crossover.
• Result: Phosphorus 7.3 mg/dL pre-study, 5.9 on calcium acetate versus 7.8 on placebo (p<0.01). Calcium 8.9 pre-study, 9.5 on calcium acetate versus 8.8 on placebo (p<0.01). A mean 19% fall in phosphorus and a 7% rise in calcium.
• Funding: industry-funded (manufacturer trials submitted for approval)

Limit of this finding: The verdict that a 7% rise in serum calcium is "clinically unimportant" is the label's own judgement, not a finding from the trial: calcium rose from 8.9 to 9.5 mg/dL against 8.8 mg/dL on placebo at p<0.01. The label also misspells "significantally" in this sentence; that misspelling is in the source and is quoted as printed. The table this comes from carries its footnotes inside the header cells, so figures should be read against the label itself.

Overall, 2 weeks of treatment with calcium acetate statistically significantally (p<0.01) decreased serum phosphorus by a mean of 19% and increased serum calcium by a statistically significant (p<0.01) but clinically unimportant mean of 7%.

Where the research disagrees

Whether calcium supplements increase the risk of heart attack

• Bolland and colleagues, BMJ 2010, patient-level and trial-level meta-analysis of 15 randomised placebo-controlled trials: Calcium supplements (without coadministered vitamin D) are associated with an increased risk of myocardial infarction. As calcium supplements are widely used these modest increases in risk of cardiovascular disease might translate into a large burden of disease in the population. A reassessment of the role of calcium supplements in the management of osteoporosis is warranted. (Source 26)
• National Osteoporosis Foundation and American Society for Preventive Cardiology, 2016 (Kopecky and colleagues), on the evidence review by Chung and colleagues, expert panel clinical guideline based on a systematic review of 4 randomised trials and 27 observational studies, funded by the National Osteoporosis Foundation: The National Osteoporosis Foundation and American Society for Preventive Cardiology adopt the position that there is moderate-quality evidence (B level) that calcium with or without vitamin D intake from food or supplements has no relationship (beneficial or harmful) to the risk for cardiovascular and cerebrovascular disease, mortality, or all-cause mortality in generally healthy adults at this time. In light of the evidence available to date, calcium intake from food and supplements that does not exceed the tolerable upper level of intake (defined by the National Academy of Medicine as 2000 to 2500 mg/d) should be considered safe from a cardiovascular standpoint. (Source 27)

How high the tolerable upper intake level for calcium is

• Institute of Medicine 2011, as reported by Ross and colleagues, reference-intake setting by a 14-member expert committee: The 1997 ULs for calcium were 2500 mg/d for all ages above 1 yr, whereas the ULs for calcium now range from 1000–3000 mg/d, depending on life-stage group (Table 1). (Source 28)
• National Osteoporosis Foundation and American Society for Preventive Cardiology 2016, describing the adult range, expert panel clinical guideline: calcium intake from food and supplements that does not exceed the tolerable upper level of intake (defined by the National Academy of Medicine as 2000 to 2500 mg/d) should be considered safe from a cardiovascular standpoint. (Source 27)

Whether calcium supplementation in pregnancy prevents pre-eclampsia

• Cochrane 2025 (Cluver and colleagues), after excluding 20 previously included trials, systematic review and random-effects meta-analysis with RoB 2 and GRADE: Calcium may result in little to no difference in pre‐eclampsia (risk ratio (RR) 0.83, 95% confidence interval (CI) 0.67 to 1.04; 6 RCTs, 15,364 women; risk difference (RD) 9/1000 fewer, 95% CI 17 fewer to 2 more; low‐certainty evidence). Sensitivity analysis excluding small studies (fewer than 500 participants) indicates little to no difference in pre‐eclampsia (RR 0.92, 95% CI 0.79 to 1.05; 4 RCTs, 14,730 women; high‐certainty evidence). (Source 5)
• The same review team on why the picture changed, trustworthiness assessment of previously included randomised trials: We excluded 20 previously included trials; 11 because eligibility criteria changed and nine because they are awaiting classification due to trustworthiness issues. (Source 23)

How much

• Reference intake: For the medicinal products, dosing is set by the label or the prescriber: the over-the-counter calcium carbonate antacid label (TUMS Ultra, effective 29 September 2026) directs adults and children 12 and over to chew 2 to 3 tablets as symptoms occur. As a nutrient the position is the Institute of Medicine’s 2011 report, which set Recommended Dietary Allowances for calcium of 700 to 1300 mg a day across life-stage groups aged at least 1 year, based on bone health. Note that these are mg of elemental calcium; one TUMS Ultra tablet of 1000 mg calcium carbonate provides 410 mg of elemental calcium. (Source 29)
• Upper limit: The Institute of Medicine’s 2011 tolerable upper intake levels for calcium range from 1000 to 3000 mg a day depending on life-stage group, set on indicators including hypercalcaemia, hypercalciuria, soft-tissue calcification and kidney stones; the 1997 upper level had been a flat 2500 mg a day above age 1. The National Osteoporosis Foundation and American Society for Preventive Cardiology describe the adult range as 2000 to 2500 mg a day. Separately, the over-the-counter antacid label sets its own limit: no more than 7 tablets in 24 hours, no more than 5 if pregnant, and not at maximum dose for more than 2 weeks without a doctor. (Source 28)
• Studied: The Women’s Health Initiative gave 1000 mg of elemental calcium as calcium carbonate with 400 IU of vitamin D3 daily for an average of 7 years. (Source 12)
• Studied: The two colorectal adenoma trials gave 1200 mg a day of supplementary elemental calcium for a mean of 4 years, and 2000 mg a day for three years. (Source 7)
• Studied: Bolland’s meta-analysis pooled placebo-controlled trials of calcium supplements of at least 500 mg a day in people of mean age over 40. (Source 17)
• Studied: The levothyroxine interaction study used calcium carbonate, calcium citrate and calcium acetate in doses each containing 500 mg elemental calcium. (Source 8)
• Studied: The antacid heartburn crossover trial used chewable tablets containing 1000 mg calcium carbonate, against gums containing 600 mg and 900 mg. (Source 2)
• Studied: The prescription phosphate binder label starts dialysis patients at 2 calcium acetate capsules with each meal, with most needing 3 to 4 capsules per meal. (Source 10)

A common belief, and what the research shows

The belief: Taking a calcium supplement every day protects your bones from breaking.

What the research shows: The largest evidence synthesis says otherwise. Pooling 33 randomised trials in 51,145 community-dwelling adults over 50, "the use of supplements that included calcium, vitamin D, or both compared with placebo or no treatment was not associated with a lower risk of fractures among community-dwelling older adults." In the 36,282-woman Women’s Health Initiative trial, calcium with vitamin D raised hip bone density by 1.06% but gave a hip-fracture hazard ratio of 0.88 (95% CI 0.72 to 1.08), and "The risk of renal calculi increased with calcium plus vitamin D (hazard ratio, 1.17; 95 percent confidence interval, 1.02 to 1.34)." A denser bone reading on a scan is not the same as fewer broken bones.

Questions and answers

What is it?

Calcium is a mineral, but as a medicine it is always a calcium salt, and the number on the front of the pack is the salt rather than the calcium itself. The over-the-counter antacid TUMS Ultra lists 1000 mg of calcium carbonate as the active ingredient in each tablet, and the same label's other-information panel puts the elemental calcium in that tablet at 410 mg. Other medicinal forms are calcium citrate (a supplement) and calcium acetate (a prescription phosphate binder). (Source 30)

What does it do in the body?

Three different jobs depending on the product. In the stomach, calcium carbonate neutralises acid, relieving heartburn for about half an hour to two hours. In the gut with a meal, calcium binds dietary phosphate into an insoluble complex that leaves in the stool, which is how calcium acetate lowers blood phosphate in kidney failure. In the body, calcium is the main mineral of bone, which is the basis the Institute of Medicine used to set reference intakes. (Source 13)

Is it good or bad for you?

It depends entirely on the purpose and the amount. As a short-course antacid it works and is cheap. As a daily bone supplement the evidence does not support it: 33 trials in 51,145 older adults found no fracture reduction. At high or prolonged intakes it causes harm - kidney stones in the WHI trial, milk-alkali syndrome from antacid overuse, and high blood calcium in dialysis patients. Whether it raises heart attack risk is genuinely disputed. (Source 28)

How do you get more of it?

As studied: trials have used calcium carbonate and calcium citrate tablets, usually 500 to 2000 mg of elemental calcium a day. The WHI gave 1000 mg of elemental calcium as calcium carbonate with 400 IU of vitamin D3. The Institute of Medicine set its reference intakes at 700 to 1300 mg a day of elemental calcium for people aged 1 and over, assuming vitamin D requirements were also met. None of this is advice about what any individual should take. (Source 29)

If it is harmful, what reduces it?

When calcium is the problem - too much of it in the blood - the first step is stopping the source. In the milk-alkali case the antacid was stopped, fluids were given intravenously and serum calcium normalised within six days. For prescription calcium acetate the label says to cut the dose or stop it, with acute haemodialysis for severe hypercalcaemia. (Source 10)

Why might someone be low in it or missing it?

Low calcium intake is why reference intakes exist at all: the Institute of Medicine set them for normal healthy people, not for anyone with a disease, and based them on calcium balance studies up to age 50 and on observational and trial evidence after that. Intake is only one route to low calcium; absorption depends on vitamin D status, and several common medicines, including levothyroxine and antibiotics in the other direction, interact with calcium in the gut. (Source 31)

Which whole foods contain it or feed it?

The Institute of Medicine based the infant reference intake on the calcium content of human breast milk, and the intakes for ages 1 to 50 on calcium balance studies, which is the committee’s own statement of where dietary calcium comes from and how it was measured. The sources we reached for this entry are drug labels and outcome trials rather than food composition tables, so we have not listed individual foods here. (Source 32)

We searched: We tried the NIH Office of Dietary Supplements calcium fact sheet and the NCBI Bookshelf copy of the 2011 Institute of Medicine report; both were blocked (Cloudflare and reCAPTCHA challenge pages). We used the open-access summary of that report by members of the IOM committee instead, which gives the reference intakes but not a food list.

What happens if you do not have it?

The Institute of Medicine committee judged that calcium has a causal role in skeletal health, which is what the reference intakes are built on, and that the evidence for everything else - cancer, cardiovascular disease, diabetes, autoimmune disease - was inconsistent and insufficient. For supplements specifically, stopping them does not appear to cost fractures: 33 trials found no fracture benefit from taking them in the first place. (Source 21)

How can you test for it?

Blood calcium is a routine, widely available test, and for anyone on prescription calcium acetate the label says to check it twice weekly while the dose is being adjusted. It measures calcium in the blood, not calcium stores in bone, and it is normal in most people whose intake is low because the body defends the blood level; bone density is measured separately by scan. The milk-alkali case was found exactly this way, by a blood test showing hypercalcaemia with metabolic alkalosis and kidney injury. (Source 10)

References

1. DailyMed, US National Library of Medicine (Structured Product Label). Calcium Acetate Capsules - DRUG INTERACTIONS (FDA label). 2026. Read the source
2. Alimentary Pharmacology & Therapeutics. Clinical effectiveness of a new antacid chewing gum on heartburn and oesophageal pH control. 2002. PMID 12452946, DOI 10.1046/j.1365-2036.2002.01380.x. Read the source
3. DailyMed, US National Library of Medicine (Structured Product Label). Calcium Acetate Capsules - CLINICAL STUDIES, placebo-controlled crossover study (FDA label). 2026. Read the source
4. JAMA. Association Between Calcium or Vitamin D Supplementation and Fracture Incidence in Community-Dwelling Older Adults: A Systematic Review and Meta-analysis. 2017. PMID 29279934, DOI 10.1001/jama.2017.19344. Read the source
5. Cochrane Database of Systematic Reviews. Calcium supplementation during pregnancy for preventing hypertensive disorders and related problems. 2025. PMID 41330480, DOI 10.1002/14651858.CD001059.pub6. Read the source
6. Cochrane Database of Systematic Reviews. Calcium supplementation for prevention of primary hypertension. 2022. PMID 35014026, DOI 10.1002/14651858.CD010037.pub4. Read the source
7. Cochrane Database of Systematic Reviews. Dietary calcium supplementation for preventing colorectal cancer and adenomatous polyps. 2008. PMID 18254022, DOI 10.1002/14651858.CD003548.pub4. Read the source
8. Thyroid. Absorption of levothyroxine when coadministered with various calcium formulations. 2011. PMID 21595516, DOI 10.1089/thy.2010.0296. Read the source
9. British Journal of Clinical Pharmacology. The influence of chronic administration of calcium carbonate on the bioavailability of oral ciprofloxacin. 1993. PMID 8471407. Read the source
10. DailyMed, US National Library of Medicine (Structured Product Label). Calcium Acetate Capsules - WARNINGS AND PRECAUTIONS, Hypercalcemia (FDA label). 2026. Read the source
11. DailyMed, US National Library of Medicine (Structured Product Label). Dicyclomine Hydrochloride Capsules and Tablets, USP label - 7 DRUG INTERACTIONS: 7.4 Effect of Antacids, 7.5 Effect on Absorption of Other Drugs, 7.6 Effect on Gastric Acid Secretion. 2026. Read the source
12. The New England Journal of Medicine. Calcium plus vitamin D supplementation and the risk of fractures. 2006. PMID 16481635, DOI 10.1056/NEJMoa055218. Read the source
13. DailyMed, US National Library of Medicine (Structured Product Label). Calcium Acetate Capsules - CLINICAL PHARMACOLOGY and Mechanism of Action (FDA label, American Health Packaging). 2026. Read the source
14. DailyMed, US National Library of Medicine (Structured Product Label). TUMS ULTRA (calcium carbonate) chewable tablet - Drug Facts, Warnings panel: "Ask a doctor or pharmacist before use if you are", "When using this product" and "Keep out of reach of children" (OTC label, Haleon US Holdings LLC). 2026. Read the source
15. DailyMed, US National Library of Medicine (Structured Product Label). TUMS ULTRA (calcium carbonate) chewable tablet - Drug Facts, Directions panel (OTC label, Haleon US Holdings LLC). 2026. Read the source
16. Irish Medical Journal. Antacid Abuse - A Case of Milk-Alkali Syndrome. 2026. PMID 42180434. Read the source
17. BMJ. Effect of calcium supplements on risk of myocardial infarction and cardiovascular events: meta-analysis. 2010. PMID 20671013, DOI 10.1136/bmj.c3691. Read the source
18. Cochrane Database of Systematic Reviews. Phosphate binders for preventing and treating chronic kidney disease-mineral and bone disorder (CKD-MBD). 2025. PMID 40576086, DOI 10.1002/14651858.CD006023.pub4. Read the source
19. JAMA. Association Between Calcium or Vitamin D Supplementation and Fracture Incidence in Community-Dwelling Older Adults (conclusions). 2017. PMID 29279934, DOI 10.1001/jama.2017.19344. Read the source
20. Annals of Internal Medicine. Calcium Intake and Cardiovascular Disease Risk: An Updated Systematic Review and Meta-analysis. 2016. PMID 27776363, DOI 10.7326/M16-1165. Read the source
21. The Journal of Clinical Endocrinology and Metabolism. The 2011 report on dietary reference intakes for calcium and vitamin D from the Institute of Medicine (conclusions on extraskeletal outcomes). 2011. PMID 21118827, DOI 10.1210/jc.2010-2704. Read the source
22. Cochrane Database of Systematic Reviews. Calcium supplementation for prevention of primary hypertension (authors’ conclusions). 2022. PMID 35014026, DOI 10.1002/14651858.CD010037.pub4. Read the source
23. Cochrane Database of Systematic Reviews. Calcium supplementation during pregnancy for preventing hypertensive disorders and related problems (included and excluded trials). 2025. PMID 41330480, DOI 10.1002/14651858.CD001059.pub6. Read the source
24. Digestion. Comparison of two antacid preparations on intragastric acidity - a two-centre open randomised cross-over placebo-controlled trial. 2007. PMID 17496418, DOI 10.1159/000102627. Read the source
25. DailyMed, US National Library of Medicine (Structured Product Label). Calcium Acetate Capsules - CLINICAL STUDIES (FDA label, American Health Packaging). 2026. Read the source
26. BMJ. Effect of calcium supplements on risk of myocardial infarction and cardiovascular events: meta-analysis (conclusions). 2010. PMID 20671013, DOI 10.1136/bmj.c3691. Read the source
27. Annals of Internal Medicine. Lack of Evidence Linking Calcium With or Without Vitamin D Supplementation to Cardiovascular Disease in Generally Healthy Adults: A Clinical Guideline From the National Osteoporosis Foundation and the American Society for Preventive Cardiology. 2016. PMID 27776362, DOI 10.7326/M16-1743. Read the source
28. The Journal of Clinical Endocrinology and Metabolism. The 2011 report on dietary reference intakes for calcium and vitamin D from the Institute of Medicine (tolerable upper intake levels). 2011. PMID 21118827, DOI 10.1210/jc.2010-2704. Read the source
29. The Journal of Clinical Endocrinology and Metabolism. The 2011 report on dietary reference intakes for calcium and vitamin D from the Institute of Medicine: what clinicians need to know (abstract). 2011. PMID 21118827, DOI 10.1210/jc.2010-2704. Read the source
30. DailyMed, US National Library of Medicine (Structured Product Label). TUMS ULTRA (calcium carbonate) chewable tablet - Drug Facts, Other information panel (OTC label, Haleon US Holdings LLC; the label prints a second, flavour-specific "Other information" panel for Tropical Fruit and Assorted Fruit that repeats the same per-tablet figures). 2026. Read the source
31. The Journal of Clinical Endocrinology and Metabolism. The 2011 report on dietary reference intakes for calcium and vitamin D from the Institute of Medicine (how the DRIs were set). 2011. PMID 21118827, DOI 10.1210/jc.2010-2704. Read the source
32. The Journal of Clinical Endocrinology and Metabolism. The 2011 report on dietary reference intakes for calcium and vitamin D from the Institute of Medicine (what the calcium DRIs are based on). 2011. PMID 21118827, DOI 10.1210/jc.2010-2704. Read the source