Magnesium salts used as medicines

What it is

These are simple magnesium compounds, each with a different medical job. Magnesium oxide tablets (a 400 mg tablet supplies 241.3 mg of elemental magnesium) and magnesium hydroxide are sold over the counter as antacids and laxatives; magnesium citrate solution is sold as a saline laxative that generally produces a bowel movement in half an hour to six hours. Magnesium sulfate given by injection is a prescription drug used to prevent and control the seizures of pre-eclampsia and eclampsia. Magnesium itself is an essential mineral involved in over 600 enzymatic reactions, with blood levels normally held between about 0.7 and 1.1 mmol/L.

What the research says

Magnesium does three quite separate things depending on the salt and the route. Swallowed and poorly absorbed, it draws water into the bowel and works as a laxative, and it neutralises stomach acid. Injected, it blocks neuromuscular transmission and suppresses seizures - and here the evidence is unusually strong: the Magpie Trial randomised 10,141 women with pre-eclampsia and halved eclampsia, with 11 fewer women fitting per 1,000 treated. Given before very preterm birth it reduces cerebral palsy in children up to two years (NNTB 60), though the school-age follow-ups no longer show a difference. For severe asthma, the biggest trial found no benefit. The dose-limiting problem throughout is that magnesium leaves the body only through the kidneys, so people with poor kidney function can accumulate it to the point of paralysis or death.

Evidence grade: Well established.

How it works

Drug class: Inorganic magnesium salts used as a saline (osmotic) laxative, as an antacid, and - given intravenously - as an anticonvulsant and neuromuscular blocking agent in obstetrics

Magnesium is a cofactor for hundreds of enzyme reactions and is needed for nerve signalling and muscle contraction. As a medicine, the injected form stops seizures by blocking transmission at the junction between nerve and muscle and reducing the acetylcholine released there; it also relaxes blood vessels, which is why flushing and sweating are the usual first side effects. Swallowed magnesium salts are poorly absorbed, so they stay in the gut, hold water there and act as a laxative, and neutralise stomach acid. (Source 1)

What it is used for

• The Magpie Trial randomised 10,141 women in 33 countries: eclampsia occurred in 0.8% on magnesium sulfate versus 1.9% on placebo, a 58% relative reduction and 11 fewer women with eclampsia per 1,000. The Cochrane review of six trials in 11,444 women gives RR 0.41 (NNTB 100) with a non-significant reduction in maternal death, no clear difference in serious maternal morbidity, and no clear difference in stillbirth or neonatal death. Evidence: established. (Source 2)
• The approved over-the-counter use is long-standing, but the randomised evidence is small. A double-blind placebo-controlled trial in 34 Japanese women with mild to moderate chronic constipation met its overall-improvement endpoint in 70.6% on magnesium oxide versus 25.0% on placebo and shortened colonic transit, but found no significant difference in complete spontaneous bowel movements. Evidence: limited. (Source 3)
• This is a traditional over-the-counter indication carried on an OTC monograph rather than on outcome trials. We did not reach any systematic review or placebo-controlled trial of magnesium-salt antacids for dyspepsia; the only source we can cite is the label, whose stated purpose is 'Antacid' with the use 'relieves acid indigestion, upset stomach' and the warning that it may have a laxative effect. Evidence: unknown. (Source 4)
• Not in the US magnesium sulfate label, but internationally recommended practice. The 2024 Cochrane review of six RCTs (5,917 women, 6,759 fetuses) found cerebral palsy reduced up to two years' corrected age (RR 0.71, NNTB 60) and death or cerebral palsy reduced (RR 0.87, NNTB 56), both high-certainty; but at early school age there may be little to no difference, and maternal adverse effects severe enough to stop treatment were about three times commoner. Evidence: established. (Source 5)
• The 3Mg trial randomised 1,109 adults with severe acute asthma in 34 UK emergency departments to intravenous magnesium sulfate, nebulised magnesium sulfate or placebo alongside standard therapy. Hospital admission rates did not differ, and the investigators concluded nebulised magnesium sulfate has no role and at best a limited role for the intravenous form. Evidence: not-supported. (Source 6)
• The label itself disclaims this use: it states that if magnesium sulfate is given for preterm labour the woman should be told the efficacy and safety of such use have not been established, and that use beyond 5 to 7 days may cause fetal abnormalities including skeletal demineralisation, osteopenia and neonatal fracture. Evidence: not-supported. (Source 7)

Interactions

• Tetracycline antibiotics (and, by the same mechanism, calcium and zinc) (pharmacokinetic study): Magnesium binds tetracyclines in the gut so less of the antibiotic is absorbed. The same review notes the traffic runs both ways: some drugs deplete the body's magnesium. (Source 8)
• Ciprofloxacin and other fluoroquinolone antibiotics (pharmacokinetic study): A magnesium- and aluminium-containing antacid taken within four hours before ciprofloxacin cut the amount absorbed to as little as 15% of normal. Giving the antacid six hours before, or two hours after, removed the effect. (Source 9)
• Cefdinir and other oral cephalosporins (pharmacokinetic study): A magnesium- or aluminium-containing antacid taken at the same time as cefdinir cuts both the speed and the amount absorbed by about 40% and delays the peak by an hour. (Source 10)
• Any other medicine taken by mouth (label): The over-the-counter laxative label puts the general rule plainly: take it two or more hours away from other drugs, because laxatives can change how other drugs work. (Source 11)
• Proton pump inhibitors, thiazide diuretics and calcineurin inhibitors (pharmacokinetic study): These drugs push magnesium out of the body or block its absorption, and are common causes of low blood magnesium - the opposite problem to taking a magnesium medicine. (Source 12)
• Alcohol (pharmacokinetic study): Alcoholism is listed among the common causes of low blood magnesium, through disturbed intestinal and renal handling of the mineral. (Source 12)
• Food and drink generally, for the laxative forms (label): The saline laxative pulls water into the bowel, so the label directs a full 8-ounce glass of liquid with each dose. (Source 13)

Stopping it

• There is no dependence or withdrawal syndrome described for magnesium salts. For the injected form the question is how long to continue: a Cochrane review of four trials in 1,713 women found shortening postpartum maintenance below 24 hours left a wide range of possible benefit or harm for eclampsia, with low-certainty evidence. (Source 14)
• The same review found shortening the regimen made little or no difference to severe morbidity or to side effects such as respiratory depression, again on low-certainty evidence. (Source 14)
• For the over-the-counter laxative, the label itself sets a stopping rule: ask a doctor before use if you have already used a laxative for longer than a week, and stop use and ask a doctor if there is rectal bleeding or no bowel movement after use. (Source 15)
• In pregnancy, duration is a safety matter rather than a comfort one: the label states continuous administration beyond 5 to 7 days can cause fetal bone abnormalities, and that the shortest harmful duration is not known. (Source 7)

What goes wrong

A quarter of women given magnesium sulfate reported side effects, against one in twenty on placebo. (Source 2)

• Randomized trial, High certainty.
• Size: 4,999 magnesium sulfate versus 4,993 placebo.
• Who: Women with pre-eclampsia in the Magpie Trial.
• How long: duration of treatment.
• Result: 1,201 of 4,999 (24%) reported side effects versus 228 of 4,993 (5%)
• Funding: not stated in the abstract.

1201 of 4999 (24%) women given magnesium sulphate reported side-effects versus 228 of 4993 (5%) given placebo.

Side effects, mainly flushing, were five times commoner on magnesium sulfate, with one extra affected woman for every six treated. (Source 16)

• Systematic review, High certainty.
• Size: Six trials, 11,444 women.
• Who: Women with pre-eclampsia.
• How long: duration of treatment.
• Result: 24% versus 5%; RR 5.26, 95% CI 4.59 to 6.03; NNTH 6, 95% CI 5 to 6.
• Funding: Cochrane review; funding not stated in abstract.

Side effects, primarily flushing, were more common with magnesium sulphate (24% versus 5%; RR 5.26, 95% CI 4.59 to 6.03; number need to treat for an additional harmful outcome (NNTH) 6, 95% CI 5 to 6).

Magnesium sulfate given for fetal neuroprotection roughly tripled maternal adverse effects severe enough to stop the infusion. (Source 17)

• Systematic review, Moderate certainty.
• Size: Three RCTs, 4,736 women.
• Who: Women at risk of preterm birth given magnesium sulfate for fetal neuroprotection.
• How long: duration of treatment.
• Result: Average RR 3.21, 95% CI 1.88 to 5.48.
• Funding: Cochrane review; funding not stated in abstract.

However, magnesium sulphate probably increased maternal adverse effects severe enough to stop treatment (average RR 3.21, 95% CI 1.88 to 5.48; 3 RCTs, 4736 women; moderate-certainty evidence).

Magnesium oxide taken as a laxative caused symptomatic and in one case fatal hypermagnesemia in older people with impaired kidneys. (Source 18)

• Case series, Very low certainty.
• Size: Four elderly patients.
• Who: Patients over 65 with constipation and renal dysfunction, several unable to report symptoms because of stroke or dementia.
• How long: not stated.
• Result: Four cases of symptomatic hypermagnesemia, one with a lethal course; the authors note serum magnesium is not usually checked in people on magnesium cathartics.
• Funding: not stated.

In this report, we present the cases of four elderly patients with constipation and symptomatic hypermagnesemia caused by MgO ingestion, one of which had a lethal course.

Continuous magnesium sulfate in pregnancy beyond 5 to 7 days can demineralise the fetal skeleton and has been followed by neonatal fracture - a label position dated 2026. (Source 7)

• Official position, Certainty not rated.
• Size: not stated; the label cites retrospective epidemiological studies and case reports.
• Who: Pregnant women given continuous magnesium sulfate.
• How long: more than 5 to 7 days.
• Result: No rate given; reported effects are fetal hypocalcemia, skeletal demineralisation, osteopenia, other skeletal abnormalities and neonatal fracture. The shortest duration of treatment that can cause fetal harm is stated to be unknown.
• Funding: manufacturer label.

FETAL HARM: Continuous administration of magnesium sulfate beyond 5-7 days to pregnant women can lead to hypocalcemia and bone abnormalities in the developing fetus. These bone abnormalities include skeletal demineralization and osteopenia. In addition, cases of neonatal fracture have been reported.

Magnesium toxicity progresses from flushing and lost reflexes to flaccid paralysis, circulatory collapse and respiratory paralysis - a label position dated 2026. (Source 19)

• Official position, Certainty not rated.
• Size: not stated.
• Who: Patients given parenteral magnesium, especially with renal impairment.
• How long: not applicable.
• Result: Deep tendon reflexes diminish above 4 mEq/L and disappear approaching 10 mEq/L, where respiratory paralysis may occur; heart block may occur at this or lower levels.
• Funding: manufacturer label.

The adverse effects of parenterally administered magnesium usually are the result of magnesium intoxication. These include flushing, sweating, hypotension, depressed reflexes, flaccid paralysis, hypothermia, circulatory collapse, cardiac and central nervous system depression proceeding to respiratory paralysis.

A magnesium- and aluminium-containing antacid taken shortly before ciprofloxacin cut the amount of the antibiotic absorbed to a fraction of normal. (Source 9)

• Blood level study, Low certainty.
• Size: Healthy male volunteers across three separate crossover studies.
• Who: Healthy male volunteers, not patients being treated for infection.
• How long: Single doses, three- or four-period crossover.
• Result: Relative bioavailability of ciprofloxacin compared with control was 15.1%, 23.2% and 70% when the antacid was given 5 to 10 minutes, 2 hours and 4 hours beforehand; antacid 6 hours before or 2 hours after did not affect absorption.
• Funding: not stated.

Limit of this finding: These figures come from healthy male volunteers given single doses in crossover studies, not from patients being treated for an infection, so the size of the effect during real treatment is not established by this paper. The source also reports two of the three percentages to one decimal place and the third as a whole number; that asymmetry is the paper's own.

Percentages of relative bioavailability compared with control values were 15.1%, 23.2%, and 70% for the 5 to 10 minute, 2 hour, and 4 hour antacid pretreatments, respectively.

What the evidence supports

Magnesium sulfate halved eclampsia in women with pre-eclampsia, with 11 fewer women fitting per 1,000 treated. (Source 2)

• Randomized trial, High certainty.
• Size: 10,141 women randomised (5,071 magnesium sulfate, 5,070 placebo), follow-up for 10,110 (99.7%)
• Who: Women in 33 countries with blood pressure 140/90 mm Hg or more and proteinuria 1+ or more, not yet delivered or 24 hours or less postpartum.
• How long: until discharge from hospital after delivery.
• Result: Eclampsia 40 (0.8%) versus 96 (1.9%); 58% lower risk, 95% CI 40-71; 11 fewer women with eclampsia per 1,000 women; maternal mortality relative risk 0.55, 0.26-1.14.
• Funding: not stated in the abstract.

Limit of this finding: The paper gives the maternal mortality figure as “relative risk 0.55, 0.26-1.14” without saying what confidence level that interval is. It labels the neighbouring estimates in the same abstract 99% CI, so do not read this one as a 95% interval. The 58 percent lower risk of eclampsia IS given with a 95% CI of 40-71.

Women allocated magnesium sulphate had a 58% lower risk of eclampsia (95% CI 40-71) than those allocated placebo (40, 0.8%, vs 96, 1.9%; 11 fewer women with eclampsia per 1000 women).

Across six placebo-controlled trials, magnesium sulfate more than halved the risk of eclampsia and reduced placental abruption. (Source 16)

• Systematic review, High certainty.
• Size: 15 trials in the review; six trials with 11,444 women in the placebo comparison.
• Who: Women with pre-eclampsia.
• How long: to hospital discharge, with one trial following to a median 26 months.
• Result: Eclampsia RR 0.41, 95% CI 0.29 to 0.58, NNTB 100 (95% CI 50 to 100); placental abruption RR 0.64, 95% CI 0.50 to 0.83, NNTB 100.
• Funding: Cochrane review; funding not stated in abstract.

Six (11,444 women) compared magnesium sulphate with placebo or no anticonvulsant: magnesium sulphate more than a halved the risk of eclampsia (risk ratio (RR) 0.41, 95% confidence interval (CI) 0.29 to 0.58; number needed to treat for an additional beneficial outcome (NNTB) 100, 95% CI 50 to 100)

Magnesium sulfate given to women at risk of preterm birth reduced cerebral palsy in their children up to two years of age. (Source 5)

• Systematic review, High certainty.
• Size: Six RCTs, 5,917 women and 6,759 fetuses alive at randomisation; 6,107 children in the cerebral palsy analysis.
• Who: Women at risk of preterm birth before 34 weeks' gestation, all trials in high-income countries.
• How long: to two years' corrected age for the primary outcomes.
• Result: Cerebral palsy RR 0.71, 95% CI 0.57 to 0.89, NNTB 60 (95% CI 41 to 158); death or cerebral palsy RR 0.87, 95% CI 0.77 to 0.98, NNTB 56 (95% CI 32 to 363)
• Funding: Cochrane review; funding not stated in abstract.

corrected age, magnesium sulphate compared with placebo reduced cerebral palsy (RR 0.71, 95% CI 0.57 to 0.89; 6 RCTs, 6107 children; number needed to treat for additional beneficial outcome (NNTB) 60, 95% CI 41 to 158) and death or cerebral palsy (RR 0.87, 95% CI 0.77 to 0.98; 6 RCTs, 6481 children; NNTB 56, 95% CI 32 to 363) (both high-certainty evidence).

Magnesium sulfate prevented eclampsia better than phenytoin or nimodipine. (Source 20)

• Systematic review, Moderate certainty.
• Size: Three trials, 2,291 women versus phenytoin; one trial, 1,650 women versus nimodipine.
• Who: Women with pre-eclampsia.
• How long: to hospital discharge.
• Result: Versus phenytoin RR 0.08, 95% CI 0.01 to 0.60; versus nimodipine RR 0.33, 95% CI 0.14 to 0.77.
• Funding: Cochrane review; funding not stated in abstract.

Limit of this finding: The same review's conclusion says magnesium sulphate 'probably reduces maternal death', but the estimate behind that phrase is RR 0.54 with a 95% confidence interval of 0.26 to 1.10, which includes no effect, and the review itself describes it elsewhere as a non-significant reduction. The eclampsia comparisons quoted in this finding are statistically significant; the maternal-death wording is not supported by its own interval.

Magnesium sulphate reduced eclampsia compared to phenytoin (three trials, 2291 women; RR 0.08, 95% CI 0.01 to 0.60) and nimodipine (one trial, 1650 women; RR 0.33, 95% CI 0.14 to 0.77).

Magnesium oxide improved overall constipation and speeded colonic transit compared with placebo in a small trial. (Source 3)

• Randomized trial, Low certainty.
• Size: 34 randomised, 33 analysed (16 placebo, 17 magnesium oxide)
• Who: Japanese women with mild to moderate chronic constipation.
• How long: 28 days.
• Result: Primary endpoint of overall improvement met by 25.0% of placebo versus 70.6% of magnesium oxide (P = 0.015); Bristol stool form and colonic transit time both improved (P < 0.001)
• Funding: not stated.

The primary endpoint was met by 25.0% of placebo vs 70.6% of MgO group (P = 0.015).

What the evidence does not support

Neither intravenous nor nebulised magnesium sulfate reduced hospital admission in adults with severe acute asthma. (Source 6)

• Randomized trial, High certainty.
• Size: 1,109 adults recruited (92% of the 1,200 planned); 332 nebulised, 394 intravenous, 358 control.
• Who: Adults aged 16 and over with severe acute asthma at 34 UK emergency departments, excluding life-threatening features.
• How long: hospital admission within 7 days; breathlessness over 2 hours.
• Result: Admission 79% nebulised, 72% intravenous, 78% control; intravenous odds ratio 0·73 (95% CI 0·51 to 1·04; p=0·083), nebulised 0·96 (0·65 to 1·40; p=0·819); change in VAS breathlessness 2·6 mm (-1·6 to 6·8; p=0·231) intravenous and -2·6 mm (-7·0 to 1·8; p=0·253) nebulised, both versus placebo.
• Funding: UK National Institute for Health Research Health Technology Assessment Programme (independent public funding)

Rates of hospital admission did not differ between patients treated with either form of MgSO(4) compared with controls or between those treated with nebulised MgSO(4) and intravenous MgSO(4).

The cerebral palsy benefit of antenatal magnesium sulfate was no longer detectable at early school age. (Source 5)

• Systematic review, Low certainty.
• Size: Two RCTs, 1,038 children for cerebral palsy at school age; two RCTs, 1,758 children for death.
• Who: Children exposed in utero to magnesium sulfate before preterm birth.
• How long: to early school age.
• Result: Cerebral palsy RR 0.99, 95% CI 0.69 to 1.41; death RR 0.82, 95% CI 0.66 to 1.02; major neurodevelopmental disability average RR 0.92, 95% CI 0.53 to 1.62 (very low certainty)
• Funding: Cochrane review; funding not stated in abstract.

At early school age, magnesium sulphate may have resulted in little to no difference in death (fetal, neonatal, or later) (RR 0.82, 95% CI 0.66 to 1.02; 2 RCTs, 1758 children); cerebral palsy (RR 0.99, 95% CI 0.69 to 1.41; 2 RCTs, 1038 children)

Preventing eclampsia with magnesium sulfate did not translate into less serious maternal illness, fewer stillbirths or fewer neonatal deaths. (Source 16)

• Systematic review, Moderate certainty.
• Size: Six trials, 11,444 women.
• Who: Women with pre-eclampsia.
• How long: to hospital discharge.
• Result: Serious maternal morbidity RR 1.08, 95% CI 0.89 to 1.32; stillbirth or neonatal death RR 1.04, 95% CI 0.93 to 1.15; caesarean section increased RR 1.05, 95% CI 1.01 to 1.10.
• Funding: Cochrane review; funding not stated in abstract.

with a non-significant reduction in maternal death (RR 0.54, 95% CI 0.26 to 1.10) but no clear difference in serious maternal morbidity (RR 1.08, 95% CI 0.89 to 1.32)

Magnesium oxide did not improve the rate of complete spontaneous bowel movements, the stricter constipation endpoint. (Source 3)

• Randomized trial, Low certainty.
• Size: 33 analysed.
• Who: Japanese women with mild to moderate chronic constipation.
• How long: 28 days.
• Result: No significant difference in complete spontaneous bowel movement response rates versus placebo (P = 0.76)
• Funding: not stated.

However, MgO did not significantly improved response rates of CSBM compared to placebo (P = 0.76).

After 16 trials in 3,020 women, there is still no compelling evidence that any one magnesium sulfate regimen beats another. (Source 21)

• Systematic review, Very low certainty.
• Size: 16 trials, 3,020 women (four trials, 409 women in eclampsia; 12 trials, 2,611 women in pre-eclampsia)
• Who: Women with pre-eclampsia or eclampsia, mostly in low- and middle-income countries.
• How long: varies by regimen compared.
• Result: No regimen shown superior; many prespecified critical outcomes were not reported in the included trials.
• Funding: Cochrane review; funding not stated in abstract.

Despite the number of trials evaluating various magnesium sulphate regimens for eclampsia prophylaxis and treatment, there is still no compelling evidence that one particular regimen is more effective than another.

Where the research disagrees

Whether the cerebral palsy benefit of antenatal magnesium sulfate lasts

• Cochrane reviewers reporting outcomes to two years' corrected age (2024), systematic review of six RCTs, 6,107 children, high-certainty evidence: corrected age, magnesium sulphate compared with placebo reduced cerebral palsy (RR 0.71, 95% CI 0.57 to 0.89; 6 RCTs, 6107 children; number needed to treat for additional beneficial outcome (NNTB) 60, 95% CI 41 to 158) and death or cerebral palsy (RR 0.87, 95% CI 0.77 to 0.98; 6 RCTs, 6481 children; NNTB 56, 95% CI 32 to 363) (both high-certainty evidence). (Source 5)
• The same reviewers reporting outcomes at early school age, two RCTs, 1,038 children, low-certainty evidence: At early school age, magnesium sulphate may have resulted in little to no difference in death (fetal, neonatal, or later) (RR 0.82, 95% CI 0.66 to 1.02; 2 RCTs, 1758 children); cerebral palsy (RR 0.99, 95% CI 0.69 to 1.41; 2 RCTs, 1038 children) (Source 5)

Whether magnesium sulfate is the drug of choice for pre-eclampsia seizures or one option among several

• FDA-approved magnesium sulfate injection label (position, 2026), regulatory label text, no study cited: However, other effective drugs are available for this purpose. (Source 22)
• Cochrane reviewers (Duley and colleagues, 2010), systematic review of 15 trials, including head-to-head comparisons in 2,291 and 1,650 women: Magnesium sulphate reduced eclampsia compared to phenytoin (three trials, 2291 women; RR 0.08, 95% CI 0.01 to 0.60) and nimodipine (one trial, 1650 women; RR 0.33, 95% CI 0.14 to 0.77). (Source 20)

How much

• Reference intake: For the injected salt, dosing is set by the prescriber, not by the reader. As a position, the 2026 magnesium sulfate injection label states that in severe pre-eclampsia or eclampsia the total initial dose is 10 to 14 g. For the over-the-counter antacid, the 2026 magnesium oxide label states one or two tablets daily; the 2026 magnesium citrate label gives 6.5 to 10 fl oz in 24 hours for adults and children 12 and over, taken with a full glass of liquid. (Source 23)
• Upper limit: As a position, the 2026 magnesium sulfate injection label states a total daily dose of 30 to 40 g should not be exceeded, falling to a maximum of 20 g per 48 hours in severe renal insufficiency. The 2026 magnesium oxide antacid label states not more than 2 tablets in a 24 hour period and warns it may have a laxative effect. (Source 23)
• Studied: The Magpie Trial gave magnesium sulfate or placebo to 10,141 women with pre-eclampsia across 33 countries; the trial's regimens varied by centre. (Source 2)
• Studied: The constipation trial gave magnesium oxide 0.5 g three times a day for 28 days. (Source 3)
• Studied: The 3Mg asthma trial gave intravenous magnesium sulfate 2 g over 20 minutes, or nebulised magnesium sulfate as three 500 mg doses in one hour. (Source 24)

A common belief, and what the research shows

The belief: Magnesium is a mineral, so a magnesium laxative or antacid is gentle and cannot do real harm.

What the research shows: Magnesium leaves the body only through the kidneys. The injection label states: Because magnesium is removed from the body solely by the kidneys, the drug should be used with caution in patients with renal impairment. A 2019 case series describes four older people with impaired kidneys who developed symptomatic hypermagnesemia from magnesium oxide taken as a laxative, one of them fatally, and notes that serum magnesium is not usually checked in people taking it. Both the magnesium oxide and magnesium citrate over-the-counter labels tell people with kidney disease to ask a doctor before use.

Questions and answers

What is it?

Magnesium is an essential mineral: part of the Mg-ATP complex, involved in over 600 enzyme reactions, and needed for energy metabolism, muscle contraction and nerve signalling. As medicines it is sold as simple salts - oxide and hydroxide as antacids and laxatives, citrate as a saline laxative, sulfate as an injection for the seizures of pre-eclampsia and eclampsia. A 400 mg magnesium oxide tablet supplies 241.3 mg of elemental magnesium. (Source 12)

What does it do in the body?

Injected, magnesium stops seizures by blocking transmission where nerve meets muscle and reducing the acetylcholine released there; it also widens blood vessels, which is why flushing and sweating come first and a blood-pressure drop at higher doses. Swallowed, the salts are poorly absorbed, so they stay in the gut, draw water in and loosen stool, and neutralise stomach acid. (Source 1)

Is it good or bad for you?

Both, and the dividing line is kidney function and dose. Given intravenously in pre-eclampsia it more than halves eclampsia - about as clear a benefit as obstetric medicine has - and the Cochrane review also says it probably reduces maternal death, although the estimate behind that phrase, RR 0.54 (95% CI 0.26 to 1.10), does not reach statistical significance. Given as a laxative to an older person with chronic kidney disease it can accumulate to the point of paralysis or death. Even in the trials where it works, a quarter of women report side effects against one in twenty on placebo. (Source 20)

How do you get more of it?

Dietary magnesium comes mainly from plant foods: in a Spanish national survey of 2,009 people the main food sources of magnesium were cereals and grains. In that same survey 79% of the whole population and 72% of plausible reporters had reported magnesium intakes below 80% of the national recommended intake. As a medicine the amount is whatever the label or prescriber directs, not something to increase on one's own; both over-the-counter labels cap the daily amount. (Source 25)

If it is harmful, what reduces it?

Magnesium is removed from the body solely by the kidneys, which is why it piles up when they are not working. For acute toxicity the label's measure is an injectable calcium salt kept immediately available to counteract the hazards of magnesium intoxication, along with watching the knee-jerk reflex and the breathing rate. For the oral forms, stopping the salt is the first step. (Source 26)

Why might someone be low in it or missing it?

Low blood magnesium develops when the gut absorbs too little or the kidneys lose too much. The common causes named in a 2024 review are alcoholism, type 2 diabetes, and medicines - proton pump inhibitors, calcineurin inhibitors and thiazide diuretics. Low dietary intake is widespread too: most of the Spanish survey population reported intakes below 80% of the recommended amount. (Source 12)

Which whole foods contain it or feed it?

Magnesium is in plant foods - in the Spanish ANIBES survey of 2,009 people the main food sources of magnesium were cereals and grains, and dairy was the main source of calcium rather than magnesium. This is intake data from one national survey using three-day dietary records, so it reflects what that population ate rather than which foods are richest in magnesium in general. (Source 25)

What happens if you do not have it?

Low blood magnesium is defined as serum magnesium below 0.7 mmol/L, outside the tightly regulated normal range of about 0.7 to 1.1 mmol/L. Not receiving magnesium as a medicine is a separate question: in the Magpie Trial the women who did not get it had eclampsia at 1.9% rather than 0.8%, and in the fetal neuroprotection trials children not exposed had more cerebral palsy, with about 60 women needing treatment to prevent one case. (Source 12)

How can you test for it?

Serum magnesium is the test in routine use, but it is a poor measure of whole-body magnesium. A review of status assessment concluded there is still no simple, rapid and accurate laboratory test for total body magnesium; serum magnesium below 0.75 mmol/L is useful for severe deficiency, and a loading test - the nearest thing to a gold standard - is needed for the grey zone between 0.75 and 0.85 mmol/L, though it is unsuitable when kidney or intestinal function is disturbed. During magnesium sulfate infusion, serum levels are monitored alongside the knee-jerk reflex and breathing rate. (Source 27)

References

1. DailyMed Structured Product Label (WG Critical Care, LLC). Magnesium Sulfate in Water for Injection - CLINICAL PHARMACOLOGY. 2026. Read the source
2. Lancet (London, England). Do women with pre-eclampsia, and their babies, benefit from magnesium sulphate? The Magpie Trial: a randomised placebo-controlled trial.. 2002. PMID 12057549, DOI 10.1016/s0140-6736(02)08778-0. Read the source
3. Journal of neurogastroenterology and motility. A Randomized Double-blind Placebo-controlled Trial on the Effect of Magnesium Oxide in Patients With Chronic Constipation.. 2019. PMID 31587548, DOI 10.5056/jnm18194. Read the source
4. DailyMed Structured Product Label (Westminster Pharmaceuticals, LLC). Magnesium Oxide Tablet (OTC) - Active Ingredient. 2026. Read the source
5. The Cochrane database of systematic reviews. Magnesium sulphate for women at risk of preterm birth for neuroprotection of the fetus.. 2024. PMID 38726883, DOI 10.1002/14651858.CD004661.pub4. Read the source
6. The Lancet. Respiratory medicine. Intravenous or nebulised magnesium sulphate versus standard therapy for severe acute asthma (3Mg trial): a double-blind, randomised controlled trial.. 2013. PMID 24429154, DOI 10.1016/S2213-2600(13)70070-5. Read the source
7. DailyMed Structured Product Label (WG Critical Care, LLC). Magnesium Sulfate in Water for Injection - WARNINGS (FETAL HARM). 2026. Read the source
8. International journal of molecular sciences. Magnesium and Drugs.. 2019. PMID 31035385, DOI 10.3390/ijms20092094. Read the source
9. Clinical pharmacology and therapeutics. Effects of aluminum and magnesium antacids and ranitidine on the absorption of ciprofloxacin.. 1989. PMID 2598571, DOI 10.1038/clpt.1989.207. Read the source
10. DailyMed Structured Product Label (Lupin Pharmaceuticals, Inc.). Cefdinir Capsules USP - PRECAUTIONS, Drug Interactions: Antacids (Aluminum- or Magnesium-Containing). 2025. Read the source
11. DailyMed Structured Product Label (Nice-Pak Products, LLC). Magnesium Citrate Oral Solution (OTC) - Ask a doctor or pharmacist before use if you are. 2026. Read the source
12. Nephrology, dialysis, transplantation : official publication of the European Dialysis and Transplant Association - European Renal Association. Magnesium biology.. 2024. PMID 38871680, DOI 10.1093/ndt/gfae134. Read the source
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