Buffered aspirin

What it is

Buffered aspirin is ordinary aspirin (acetylsalicylic acid) formulated together with antacid ingredients. The US FDA's over-the-counter monograph for internal analgesics defines it as aspirin buffered with permitted antacid ingredients so that the finished product carries at least 1.9 milliequivalents of acid-neutralising capacity per 325 mg of aspirin. A marketed 325 mg tablet is buffered with calcium carbonate, magnesium carbonate and magnesium oxide and supplies 35 mg calcium and 45 mg magnesium per tablet. The buffer changes how fast the drug is absorbed and the pH of the urine, but the active drug is the same acetylsalicylic acid.

What the research says

Two separate bodies of evidence have to be kept apart here. The aspirin outcome literature is very large: pooled randomised trials show a clear absolute benefit in people who already have occlusive vascular disease, a small and contested benefit in people who do not, and a consistent increase in major bleeding. The literature on the buffered formulation itself is thin, and what exists does not show that buffering makes aspirin safer for the stomach: a 550-case study of upper gastrointestinal bleeding found buffered aspirin carried about the same raised risk as plain aspirin. None of the large primary-prevention trials (ASPREE, ARRIVE, ASCEND) used a buffered product, so their results cannot be read as evidence about the buffered form specifically.

Evidence grade: Well established.

How it works

Drug class: Salicylate non-steroidal anti-inflammatory drug and irreversible cyclo-oxygenase-1 inhibitor (antiplatelet agent), formulated with antacid buffers

Aspirin works by permanently acetylating cyclo-oxygenase-1 inside platelets, at the amino acid serine-529. A platelet cannot make new enzyme, so a single daily dose keeps thromboxane production suppressed for the life of the platelet; this is what thins the blood. The same enzyme blockade in the stomach lining removes prostaglandins that protect the mucosa, which is why bleeding and ulceration are the characteristic harms. The antacid buffer neutralises stomach acid and speeds gastric emptying, but it does not change the enzyme blockade. (Source 1)

What it is used for

• This is what the marketed buffered tablet is actually labelled for, and the FDA monograph permits buffered aspirin as an over-the-counter analgesic active. We found no trial of the buffered formulation against plain aspirin for pain or fever, so the labelled use rests on the monograph position rather than on formulation-specific outcome data. Evidence: limited. (Source 2)
• In 16 secondary-prevention trials pooled by the Antithrombotic Trialists' Collaboration, serious vascular events fell from 8.2% to 6.7% per year on aspirin. This is the strongest part of the aspirin evidence base. The trials used plain or enteric-coated aspirin, not buffered aspirin; FDA's professional labelling lists the vascular indications without distinguishing formulation. Evidence: established. (Source 3)
• Three large 2018 trials and a 13-trial meta-analysis all point the same way: a small reduction in cardiovascular events (number needed to treat 265) sitting alongside a similar-sized increase in major bleeding (number needed to harm 210). ASPREE in healthy older adults found no significant cardiovascular benefit at all and a 38% higher rate of major haemorrhage. Evidence: disputed. (Source 4)
• FDA's professional labelling for aspirin lists these rheumatologic indications and says aspirin controlled indices of clinical disease activity in clinical studies. We did not reach those underlying studies, and the doses involved (3 g a day and above) are far higher than analgesic or antiplatelet doses. Evidence: limited. (Source 5)
• Twenty-year follow-up of five randomised trials found aspirin for five years or more reduced fatal colorectal cancer in absolute terms by 1.76%, with the benefit concentrated in the proximal colon. This was a post-hoc analysis of trials designed for vascular endpoints, and it is not a labelled use. Evidence: limited. (Source 6)
• A 2025 Cochrane review of five trials (211 participants) found aspirin after desensitisation may improve sinonasal quality of life at six months on low-certainty evidence, with everything else inconclusive and seven participants in the active arm dropping out with gastrointestinal disorders. Evidence: limited. (Source 7)

Interactions

• Alcohol (label): Drinking heavily while taking aspirin raises the risk of stomach bleeding. The over-the-counter label singles out three or more alcoholic drinks a day as a risk factor, and a case-control study of bleeding found drinking roughly doubled the risk independently of NSAID use. (Source 8)
• Alcohol (observational data) (observational study (case-control)): In the American College of Gastroenterology bleeding registry study, drinkers had about twice the risk of gastrointestinal bleeding, and the authors suggested NSAIDs and alcohol may combine. Limit: Blot and McLaughlin 2000 is a case-control study, so it can show that drinkers in the registry had more gastrointestinal bleeding but cannot show that alcohol and the drug combined to cause it. The interaction vocabulary in our brief offers no value for an observational safety study, so the design is named here in full rather than forced into the nearest listed value. (Source 9)
• Warfarin and heparin (label): Aspirin pushes warfarin off its protein binding sites, lengthening both the prothrombin time and the bleeding time, and increases heparin's anticoagulant effect. Pooled randomised trials found adding aspirin to a vitamin K antagonist more than doubled intracranial haemorrhage. (Source 10)
• Methotrexate (label): Salicylate slows the kidneys' clearance of methotrexate, which can push methotrexate levels high enough to damage the bone marrow, especially in older people or people with reduced kidney function. (Source 10)
• Other NSAIDs (ibuprofen, naproxen and others) (label): Taking aspirin with another NSAID is to be avoided: it can increase bleeding or reduce kidney function. The over-the-counter label lists taking other prescription or non-prescription NSAIDs as a stomach-bleeding risk factor. (Source 10)
• Diuretics (label): Aspirin blocks prostaglandins in the kidney, cutting renal blood flow and causing salt and fluid retention, which can blunt how well a diuretic works in someone with kidney or heart disease. The over-the-counter label tells people taking a diuretic to ask a doctor first. (Source 10)
• ACE inhibitors (label): Aspirin may reduce the sodium-lowering and blood-pressure-lowering effects of ACE inhibitors through its indirect effect on the renin-angiotensin pathway. (Source 10)
• The antacid buffer itself (urinary alkalinisation) (pharmacokinetic study): A single dose of the antacid buffer in a buffered aspirin solution measurably shifted urine pH and the rate at which the kidneys cleared salicylate, and changed how fast the aspirin was absorbed. This is the one interaction specific to the buffered formulation rather than to aspirin in general. (Source 11)
• Ginkgo biloba (clinical trial): A randomised trial in older adults already taking 325 mg of aspirin a day found adding a high dose of Ginkgo biloba extract for four weeks made no detectable difference to platelet function or bleeding reports. This is reassuring but small and short. (Source 12)
• Fish oil (omega-3 fatty acids) (clinical trial): In a crossover trial, 21 days of 8 g of omega-3 fatty acids did not make a single 325 mg aspirin dose prolong bleeding time more than olive oil did. (Source 13)
• Willow bark and other salicylate-bearing botanicals (theoretical): Willow bark supplements deliver salicylate of their own. A United States Pharmacopeia safety review calculated that 240 mg of salicin from willow bark could yield 113 mg of salicylic acid, more than an 81 mg aspirin tablet delivers, and said taking it with other salicylate-containing medicines raises the bleeding, pregnancy and Reye's syndrome risks. (Source 14)
• Prescription drugs for gout, diabetes or arthritis (label): The over-the-counter buffered aspirin label tells people taking a prescription drug for gout, diabetes or arthritis to ask a doctor or pharmacist before use. FDA's professional labelling adds that moderate aspirin doses may increase the effect of oral hypoglycaemic drugs and that salicylates antagonise uricosuric agents. (Source 10)
• Acetazolamide (label): Aspirin and acetazolamide compete for secretion at the renal tubule, so taking them together can push acetazolamide levels into the toxic range. (Source 10)

Stopping it

• In a Swedish cohort of 601,527 long-term low-dose aspirin users, stopping aspirin in the absence of major surgery or bleeding was associated with a more than 30% higher rate of cardiovascular events, with the increase appearing shortly after discontinuation and not fading over time. This is observational and cannot show causation, but it is the largest stopping dataset available. (Source 15)
• There is no withdrawal syndrome to taper off. FDA's professional labelling records that aspirin is nonnarcotic with no known potential for addiction. The question on stopping is whether the antiplatelet protection is still worth the bleeding risk, not dependence. (Source 16)
• For the over-the-counter analgesic use, the label itself sets a stopping point: stop and ask a doctor if pain gets worse or lasts more than 10 days, or if fever gets worse or lasts more than 3 days. (Source 8)

What goes wrong

In the same trial major haemorrhage was significantly more common on aspirin. (Source 17)

• Randomized trial, High certainty.
• Size: 19,114 participants.
• Who: healthy community-dwelling adults aged 70 or older.
• How long: median 4.7 years.
• Result: major haemorrhage 8.6 vs 6.2 events per 1000 person-years, HR 1.38 (95% CI 1.18-1.62; P<0.001)
• Funding: National Institute on Aging and others.

The rate of major hemorrhage was 8.6 events per 1000 person-years and 6.2 events per 1000 person-years, respectively (hazard ratio, 1.38; 95% CI, 1.18 to 1.62; P<0.001).

Buffered aspirin carried a roughly three-fold raised risk of major upper gastrointestinal bleeding, no lower than plain aspirin, and the highest relative risk of the three forms at doses above 325 mg. (Source 18)

• Case-control study, Moderate certainty.
• Size: 550 incident cases of upper gastrointestinal bleeding and 1,202 population controls.
• Who: people admitted to hospital with melaena or haematemesis confirmed by endoscopy, interviewed about aspirin use in the previous 7 days, versus controls from population census lists.
• How long: exposure window of the 7 days before bleeding onset or interview.
• Result: relative risk of upper gastrointestinal bleeding at 325 mg daily or less: plain 2.6, enteric-coated 2.7, buffered 3.1; above 325 mg daily: plain 5.8, buffered 7.0.
• Funding: not stated in the abstract.

Limit of this finding: A case-control design with interview-based exposure, so recall and confounding by indication cannot be excluded; the paper reports relative risks without confidence intervals in the abstract.

At doses greater than 325 mg, the relative risk was 5.8 for plain and 7.0 for buffered aspirin; there were insufficient data to evaluate enteric-coated aspirin at this dose level.

At over-the-counter doses aspirin and other NSAIDs were associated with a two- to three-fold increase in gastrointestinal bleeding, and drinking doubled the risk independently. (Source 9)

• Case-control study, Low certainty.
• Size: 627 patients in the American College of Gastroenterology gastrointestinal bleeding registry and 590 procedure-matched controls.
• Who: patients with gastrointestinal bleeding versus procedure-matched controls, asked about analgesic use in the past week.
• How long: exposure window of the past week.
• Result: risk of gastrointestinal bleeding increased 2-3 fold among recent users of aspirin, ibuprofen and other NSAIDs at over-the-counter doses, dose-related; no excess among paracetamol users; doubled risk among drinkers.
• Funding: not stated in the abstract.

Limit of this finding: The authors themselves say the results are not definitive and call for further research; the paper reports odds ratios only descriptively in the abstract.

Risk of GI bleeding was increased 2-3 fold among recent users of aspirin, ibuprofen and other nonsteroidal anti-inflammatory drugs (NSAIDs) at OTC doses, with risk increasing in a dose-related manner.

Adding aspirin to a vitamin K antagonist more than doubled the frequency of intracranial haemorrhage in randomised trials. (Source 19)

• Meta-analysis, Low certainty.
• Size: 3,874 participants across six randomised clinical trials.
• Who: patients on oral anticoagulation randomised to added aspirin or not.
• How long: trial durations not stated in the abstract.
• Result: intracranial haemorrhage relative risk 2.4 (95% CI 1.2-4.8, p=0.02)
• Funding: not stated in the abstract.

Limit of this finding: The authors present this as a hypothesis needing confirmation and say the magnitude of the effect is uncertain.

Use of aspirin with oral anticoagulants was associated with more than double the frequency of intracranial hemorrhage (relative risk = 2.4, 95% CI = 1.2-4.8, p = 0.02).

In pooled studies about 9% of adults with asthma had NSAID-exacerbated respiratory disease, and the mean provoking dose of aspirin was 85.8 mg. (Source 20)

• Meta-analysis, Moderate certainty.
• Size: pooled studies using blinded placebo-controlled oral provocation challenge tests or self-reported history; participant totals not given in the abstract.
• Who: adults with asthma.
• How long: single-challenge and cross-sectional data.
• Result: prevalence of NSAID-exacerbated respiratory disease 9.0% (95% CI 6-12%) by challenge and 9.9% (95% CI 9.4-10.5%) by questionnaire; mean provocative dose of aspirin 85.8 mg (95% CI 73.9-97.6); uncontrolled asthma RR 1.96 (1.25-3.07); asthma hospitalisation RR 1.37 (1.12-1.67)
• Funding: not stated in the abstract.

The prevalence of NERD in adults with asthma was 9.0% (95% CI 6-12%) using OPCTs and 9.9% (95% CI 9.4-10.5%) using self-reported history from questionnaires. The MPDA in adults with NERD was 85.8 mg (95% CI 73.9-97.6).

In children and teenagers, aspirin exposure before the onset of Reye's syndrome was far more common in cases than in matched controls, and the association was not explained by five tested sources of bias. (Source 21)

• Case-control study, Moderate certainty.
• Size: 24 case subjects and 48 matched controls.
• Who: children with Reye's syndrome and matched controls, including an alternate-condition control group.
• How long: case-control, exposure before onset of illness.
• Result: 88% of cases versus 17% of controls had received aspirin before onset; matched odds ratio 35 (95% CI 4.2 to 288)
• Funding: not stated in the abstract.

Limit of this finding: Very small study with an extremely wide confidence interval. The published abstract prints the figure as 'Eight-eight percent', an apparent typographical error for eighty-eight percent; we have not silently corrected it.

Eight-eight percent of case subjects and only 17% of controls had received aspirin prior to the onset of Reye's syndrome (matched odds ratio, 35; 95% confidence interval, 4.2 to 288).

Salicylate's ototoxic properties are well established, with tinnitus and a sensory hearing loss reported when high doses are given. (Source 22)

• Expert review, not systematic, Low certainty.
• Size: not applicable; a narrative review of animal, cell-culture and human work.
• Who: animal models and human listeners.
• How long: acute and chronic dosing studies.
• Result: no pooled effect size; the review reports frequency-dependent reductions in distortion-product otoacoustic emissions and compound action potentials in animals, and decreased word discrimination and temporal integration in humans.
• Funding: not stated in the abstract.

Limit of this finding: This is a narrative review with no stated search method, and it reports no doses, no rates and no pooled effect. Most of the mechanism it describes comes from animal and cell-culture work, so treat it as background rather than as a measured human risk. The review is about salicylate and aspirin in general, not about buffered aspirin products.

Salicylate's ototoxic properties have been well established, inducing tinnitus and a sensory hearing loss when administered in high doses.

FDA's professional labelling lists tinnitus and hearing loss among aspirin's adverse reactions, and warns that in people with existing high-frequency hearing loss tinnitus cannot be used as a warning sign of salicylate poisoning. (Source 23)

• Official position, Certainty not rated.
• Size: not applicable.
• Who: people taking aspirin, per the monograph.
• How long: not applicable.
• Result: no rates are given; the monograph presents an unranked list of reported reactions and states that many are dose-related.
• Funding: not applicable (regulatory document)

Limit of this finding: This list carries no frequencies at all, so it cannot tell a reader how common any of these are. It is a position dated 10.14.2022, not evidence.

Special Senses: Hearing loss, tinnitus. Patients with high frequency hearing loss may have difficulty perceiving tinnitus.

Stopping long-term low-dose aspirin outside surgery or bleeding was associated with a 37% higher rate of cardiovascular events. (Source 24)

• Cohort study, Low certainty.
• Size: 601,527 users of low-dose aspirin; 62,690 cardiovascular events.
• Who: Swedish prescription-register users of low-dose aspirin for primary or secondary prevention, over 40, cancer-free, with at least 80% adherence in the first observed year.
• How long: median 3.0 years of follow-up.
• Result: multivariable-adjusted HR 1.37 (95% CI 1.34-1.41); one additional cardiovascular event per year for every 74 patients who discontinued.
• Funding: not stated in the abstract.

Limit of this finding: Observational: people stop aspirin for reasons that themselves predict events, so this association must not be read as showing that stopping causes the events.

Patients who discontinued aspirin had a higher rate of cardiovascular events than those who continued (multivariable-adjusted hazard ratio, 1.37; 95% confidence interval, 1.34-1.41), corresponding to an additional cardiovascular event observed per year in 1 of every 74 patients who discontinue aspirin.

People on sodium-retaining states are advised by FDA to avoid sodium-containing buffered aspirin specifically, because of the sodium the buffer carries. (Source 10)

• Official position, Certainty not rated.
• Size: not applicable.
• Who: patients with congestive heart failure or renal failure.
• How long: not applicable.
• Result: no rates given.
• Funding: not applicable (regulatory document)

Limit of this finding: This is the one place the monograph treats the buffered form differently from plain aspirin, and it is a harm, not a benefit. The marketed calcium/magnesium-buffered tablet we read is not a sodium-containing buffer; effervescent sodium bicarbonate products are.

Sodium Restricted Diets: Patients with sodium-retaining states, such as congestive heart failure or renal failure, should avoid sodium-containing buffered aspirin preparations because of their high sodium content.

In the same Cochrane review, seven participants in the active treatment arm developed a gastrointestinal disorder and dropped out. (Source 7)

• Systematic review, Very low certainty.
• Size: 129 participants across the studies reporting adverse events.
• Who: adults with NSAID-exacerbated respiratory disease and chronic rhinosinusitis with nasal polyps, in tertiary care centres.
• How long: six and 36 months.
• Result: seven active-arm participants developed a gastrointestinal disorder and withdrew (129 participants, very low-certainty evidence); no denominator for the comparison arm is given in this passage.
• Funding: all but one study reported source of funding.

Limit of this finding: The review rates this very low certainty and the passage gives no placebo-arm comparison, so it shows that withdrawals happened, not how much more likely they are on aspirin. The doses used were desensitisation doses in specialist care, and no included study used a buffered product.

Seven participants in the active treatment group developed a gastrointestinal disorder and dropped out (129 participants, very low-certainty evidence).

What the evidence supports

In people who already had occlusive vascular disease, aspirin cut serious vascular events from 8.2% to 6.7% per year. (Source 3)

• Meta-analysis, High certainty.
• Size: 17,000 individuals across 16 secondary prevention trials, 43,000 person-years, 3,306 serious vascular events.
• Who: people at high average vascular risk who had already had an occlusive vascular event.
• How long: long-term scheduled treatment periods.
• Result: serious vascular events 6.7% vs 8.2% per year (p<0.0001); total stroke 2.08% vs 2.54% per year (p=0.002); coronary events 4.3% vs 5.3% per year (p<0.0001)
• Funding: UK Medical Research Council, British Heart Foundation, Cancer Research UK and the European Community Biomed Programme.

Limit of this finding: The pooled trials used plain or enteric-coated aspirin. No buffered-formulation trial contributed, so this does not establish the buffered form as equivalent in outcome.

In the secondary prevention trials, aspirin allocation yielded a greater absolute reduction in serious vascular events (6.7%vs 8.2% per year, p<0.0001)

Over 20 years, allocation to aspirin for five years or more reduced fatal colorectal cancer by 1.76% in absolute terms. (Source 6)

• Meta-analysis, Moderate certainty.
• Size: 14,033 patients in four trials of aspirin versus control, plus one dose-comparison trial; 391 (2.8%) developed colorectal cancer.
• Who: participants in primary and secondary vascular prevention trials.
• How long: mean 6.0 years of scheduled treatment, median 18.3 years of follow-up.
• Result: colon cancer incidence HR 0.76 (0.60-0.96, p=0.02), mortality HR 0.65 (0.48-0.88, p=0.005); proximal colon incidence HR 0.45 (0.28-0.74, p=0.001); absolute reduction 1.76% (0.61-2.91; p=0.001) in 20-year risk of any fatal colorectal cancer after 5 years of 75-300 mg daily.
• Funding: None (as stated by the authors)

Limit of this finding: Colorectal cancer was not the endpoint these trials were designed for, and the paper itself reports a higher risk of fatal colorectal cancer on 30 mg than on 283 mg daily in one trial (odds ratio 2.02, 0.70-6.05, p=0.15), which points the opposite way to the no-extra-benefit-above-75-mg conclusion.

There was no increase in benefit at doses of aspirin greater than 75 mg daily, with an absolute reduction of 1·76% (0·61-2·91; p=0·001) in 20-year risk of any fatal colorectal cancer after 5-years scheduled treatment with 75-300 mg daily.

FDA's professional labelling for aspirin states it is nonnarcotic with no known addiction potential. (Source 16)

• Official position, Certainty not rated.
• Size: not applicable.
• Who: not applicable.
• How long: not applicable.
• Result: no quantitative claim is made.
• Funding: not applicable (regulatory document)

Limit of this finding: A regulatory statement dated 10.14.2022, not a study. We found no withdrawal-syndrome literature for aspirin; the risk on stopping is loss of antiplatelet protection, not withdrawal.

Aspirin is nonnarcotic. There is no known potential for addiction associated with the use of aspirin.

What the evidence does not support

In healthy older adults, low-dose enteric-coated aspirin did not significantly reduce cardiovascular disease. (Source 17)

• Randomized trial, High certainty.
• Size: 19,114 participants (9,525 aspirin, 9,589 placebo)
• Who: community-dwelling men and women in Australia and the United States aged 70 or older (65 or older among US blacks and Hispanics) without cardiovascular disease, dementia or disability.
• How long: median 4.7 years.
• Result: cardiovascular disease 10.7 vs 11.3 events per 1000 person-years, HR 0.95 (95% CI 0.83-1.08)
• Funding: National Institute on Aging and others.

Limit of this finding: The trial used 100 mg of enteric-coated aspirin, not a buffered product.

the rate of cardiovascular disease was 10.7 events per 1000 person-years in the aspirin group and 11.3 events per 1000 person-years in the placebo group (hazard ratio, 0.95; 95% confidence interval [CI], 0.83 to 1.08)

ARRIVE found no reduction in first cardiovascular events in people at moderate estimated risk, and more gastrointestinal bleeding. (Source 25)

• Randomized trial, Moderate certainty.
• Size: 12,546 patients (6,270 aspirin, 6,276 placebo) at 501 sites in seven countries.
• Who: men aged 55 and over and women aged 60 and over with a moderate estimated cardiovascular risk, excluding people at high bleeding risk and people with diabetes.
• How long: median 60 months.
• Result: primary endpoint 269 (4.29%) vs 281 (4.48%), HR 0.96 (95% CI 0.81-1.13; p=0.6038); gastrointestinal bleeding 61 (0.97%) vs 29 (0.46%), HR 2.11 (95% CI 1.36-3.28; p=0.0007)
• Funding: industry-funded (Bayer)

Limit of this finding: The investigators state the event rate was much lower than expected, so the trial could not answer the question it was designed for; and it used enteric-coated, not buffered, aspirin.

the primary endpoint occurred in 269 (4·29%) patients in the aspirin group versus 281 (4·48%) patients in the placebo group (hazard ratio [HR] 0·96; 95% CI 0·81-1·13; p=0·6038)

The same trial found no reduction in gastrointestinal tract cancer or in all cancers over a mean 7.4 years. (Source 26)

• Randomized trial, Moderate certainty.
• Size: 15,480 participants.
• Who: adults with diabetes and no evident cardiovascular disease.
• How long: mean 7.4 years.
• Result: gastrointestinal tract cancer 157 (2.0%) vs 158 (2.0%); all cancers 897 (11.6%) vs 887 (11.5%)
• Funding: British Heart Foundation and others.

Limit of this finding: The investigators state that long-term follow-up for cancer outcomes was planned, so this is an interim picture at a follow-up much shorter than the 20 years over which Rothwell saw an effect.

There was no significant difference between the aspirin group and the placebo group in the incidence of gastrointestinal tract cancer (157 participants [2.0%] and 158 [2.0%], respectively) or all cancers (897 [11.6%] and 887 [11.5%])

The authors of that study concluded the belief that buffered aspirin is gentler than plain aspirin may be wrong. (Source 27)

• Case-control study, Moderate certainty.
• Size: 550 cases and 1,202 controls.
• Who: as above.
• How long: as above.
• Result: a three-fold increase in risk of major upper gastrointestinal bleeding with low doses of enteric-coated or buffered aspirin.
• Funding: not stated in the abstract.

Limit of this finding: The authors' phrase "three-fold" rounds their own relative risks of 2.7 for enteric-coated and 3.1 for buffered aspirin at 325 mg a day or less, so the quoted wording is more generous than the figures behind it. The abstract gives no confidence intervals for those risks, so the precision of the estimate cannot be judged from it. This is a case-control study, which can show an association but cannot establish that the formulation caused the bleeding.

Use of low doses of enteric-coated or buffered aspirin carries a three-fold increase in the risk of major UGIB. The assumption that these formulations are less harmful than plain aspirin may be mistaken.

Where the evidence is mixed

In primary prevention the same collaboration reported a 12% proportional reduction in serious vascular events, alongside crude yearly rates of 0.51% on aspirin and 0.57% on control. (Source 3)

• Meta-analysis, High certainty.
• Size: 95,000 individuals across six primary prevention trials, 660,000 person-years, 3,554 serious vascular events.
• Who: individuals at low average cardiovascular risk with no known vascular disease.
• How long: long-term scheduled treatment periods.
• Result: serious vascular events 0.51% vs 0.57% per year (p=0.0001); non-fatal myocardial infarction 0.18% vs 0.23% per year (p<0.0001); stroke 0.20% vs 0.21% per year (p=0.4); vascular mortality 0.19% vs 0.19% per year (p=0.7)
• Funding: UK Medical Research Council, British Heart Foundation, Cancer Research UK and the European Community Biomed Programme.

Limit of this finding: The paper's 12% is its rate-ratio estimate; it does not follow from the two crude rates printed in the same sentence, because 0.51% against 0.57% is a difference of about 10.5%. A reader who tries to check the 12% against those two percentages will not arrive at it. Read the 12% as the collaboration's modelled estimate, not as arithmetic you can reproduce from the quoted figures. Every trial in this meta-analysis used plain or enteric-coated aspirin, never a buffered product.

In the primary prevention trials, aspirin allocation yielded a 12% proportional reduction in serious vascular events (0.51% aspirin vs 0.57% control per year, p=0.0001), due mainly to a reduction of about a fifth in non-fatal myocardial infarction (0.18%vs 0.23% per year, p<0.0001).

A 13-trial meta-analysis put the absolute primary-prevention benefit at 0.38% (number needed to treat 265) and the absolute bleeding harm at 0.47% (number needed to harm 210). (Source 4)

• Meta-analysis, Moderate certainty.
• Size: 164,225 participants across 13 trials, 1,050,511 participant-years.
• Who: adults with no known cardiovascular disease, median age 62 years, 30,361 of 164,225 had diabetes.
• How long: at least 12 months, median baseline primary cardiovascular risk 9.2%.
• Result: composite cardiovascular outcome HR 0.89 (95% credible interval 0.84-0.95), absolute risk reduction 0.38% (95% CI 0.20%-0.55%), NNT 265; major bleeding HR 1.43 (95% credible interval 1.30-1.56), absolute risk increase 0.47% (95% CI 0.34%-0.62%), NNH 210.
• Funding: not stated in the abstract.

Limit of this finding: The abstract prints “30 361 (19%) had diabetes”, but 30,361 out of 164,225 participants is 18.5%, which rounds to 18% and not 19%. That is JAMA's own arithmetic slip and we have left its wording untouched in the quoted passage, and given the two counts here instead of a percentage. It is a slip in the description of who was studied and it does not affect the benefit and bleeding figures quoted. Note also that these are pooled trial averages: the reduction in cardiovascular events (0.38%) and the increase in major bleeding (0.47%) come from the same analysis and neither tells any one person what will happen to them.

absolute risk reduction, 0.38% [95% CI, 0.20%-0.55%]; number needed to treat, 265). Aspirin use was associated with an increased risk of major bleeding events compared with no aspirin (23.1 per 10 000 participant-years with aspirin and 16.4 per 10 000 participant-years with no aspirin) (HR, 1.43 [95% credible interval, 1.30-1.56]; absolute risk increase, 0.47% [95% CI, 0.34%-0.62%]; number needed to harm, 210).

In people with diabetes and no vascular disease, aspirin lowered serious vascular events and raised major bleeding by a similar order of magnitude. (Source 26)

• Randomized trial, High certainty.
• Size: 15,480 participants randomised.
• Who: adults with diabetes mellitus and no evident cardiovascular disease.
• How long: mean 7.4 years.
• Result: serious vascular events 658 (8.5%) vs 743 (9.6%), rate ratio 0.88 (95% CI 0.79-0.97; P=0.01); major bleeding 314 (4.1%) vs 245 (3.2%), rate ratio 1.29 (95% CI 1.09-1.52; P=0.003)
• Funding: British Heart Foundation and others.

serious vascular events occurred in a significantly lower percentage of participants in the aspirin group than in the placebo group (658 participants [8.5%] vs. 743 [9.6%]; rate ratio, 0.88; 95% confidence interval [CI], 0.79 to 0.97; P=0.01). In contrast, major bleeding events occurred in 314 participants (4.1%) in the aspirin group, as compared with 245 (3.2%) in the placebo group (rate ratio, 1.29; 95% CI, 1.09 to 1.52; P=0.003), with most of the excess being gastrointestinal bleeding and other extracranial bleeding.

Buffering changes aspirin pharmacokinetics: less intact aspirin reaches the circulation from a buffered effervescent solution, and a single antacid-buffered dose measurably changes urine pH and the renal clearance of salicylate. (Source 11)

• Blood level study, Low certainty.
• Size: 12 fasting normal volunteers.
• Who: healthy fasting adults in a complete crossover study of three 325 mg dosage forms.
• How long: plasma and urine sampling over 10 hours.
• Result: approximately 25% more aspirin reached the general circulation intact from the unbuffered tablet than from the two buffered solutions, while total salicylate absorbed was equivalent; sodium bicarbonate-citrate buffer gave significantly faster absorption than potassium bicarbonate-citrate buffer and unbuffered tablets.
• Funding: not stated in the abstract.

Limit of this finding: A 12-person single-dose study of effervescent buffered solutions, not of a buffered tablet taken long term, and it measured blood levels, not clinical outcomes.

The area under the curve and urine accumulation comparisons suggested that approximately 25% more aspirin reaches the general circulation intact after administration of the unbuffered tablet than the two solutions, but that the total salicylate absorbed is equivalent for all three dosage forms.

A Cochrane review found that aspirin treatment after desensitisation may improve sinonasal quality of life in NSAID-exacerbated respiratory disease, on low-certainty evidence. (Source 7)

• Systematic review, Low certainty.
• Size: five studies, 211 participants enrolled and 146 analysed.
• Who: adults with NSAID-exacerbated respiratory disease and chronic rhinosinusitis with nasal polyps, in tertiary care centres.
• How long: six and 36 months.
• Result: SNOT score mean difference -0.54 (95% CI -0.76 to -0.31; 3 studies, 85 participants; low-certainty evidence), equal to an 11.9-point reduction on SNOT-22 against a 9.0-point minimum clinically important difference.
• Funding: all but one study reported source of funding.

Limit of this finding: Only three small studies, 85 participants in total, sit behind this estimate, and the review rates it low certainty, so the size of the benefit is not settled. This is aspirin treatment after desensitisation in a specialist setting, not ordinary over-the-counter use, and none of the included studies used a buffered product.

ATAD may improve health-related quality of life, assessed with Sino-Nasal Outcome Test (SNOT) scores (mean difference (MD) -0.54, 95% confidence interval (CI) -0.76 to -0.31; 3 studies, 85 participants; minimum clinically important difference (MCID) 9.0 points for total score; low-certainty evidence).

Where the research disagrees

Whether buffering makes aspirin safer for the stomach

• Kelly and colleagues, Lancet 1996, case-control study of 550 bleeding cases and 1,202 controls: Use of low doses of enteric-coated or buffered aspirin carries a three-fold increase in the risk of major UGIB. The assumption that these formulations are less harmful than plain aspirin may be mistaken. (Source 27)
• Murray and colleagues, Gut 1996, on the related salicylate calcium carbasalate, endoscopist-blinded randomised crossover trial in 20 healthy volunteers, measuring erosions rather than bleeding events: It is concluded that ECC causes significantly less gastroduodenal mucosal damage than ASA administered at bioequivalent doses (Source 28)

Whether aspirin is worth taking for primary prevention

• Antithrombotic Trialists' Collaboration, Lancet 2009, meta-analysis of individual participant data from six primary prevention trials, 95,000 individuals: In primary prevention without previous disease, aspirin is of uncertain net value as the reduction in occlusive events needs to be weighed against any increase in major bleeds. (Source 29)
• ASPREE investigators, New England Journal of Medicine 2018, randomised placebo-controlled trial in 19,114 healthy adults aged 70 and over: The use of low-dose aspirin as a primary prevention strategy in older adults resulted in a significantly higher risk of major hemorrhage and did not result in a significantly lower risk of cardiovascular disease than placebo. (Source 30)

How much

• Reference intake: Dose is set by the prescriber for antiplatelet and anti-inflammatory use. As a dated position, FDA's professional labelling for aspirin gives 50-325 mg once a day for ischaemic stroke and TIA, 75-325 mg once a day for prevention of recurrent myocardial infarction, unstable angina and chronic stable angina, and 160-162.5 mg for suspected acute myocardial infarction. The over-the-counter buffered 325 mg tablet is labelled as 2 tablets every 4 hours for adults and children 12 and over. None of this is a recommendation to any reader. (Source 31)
• Upper limit: The marketed buffered 325 mg tablet's label sets a maximum of 12 tablets in 24 hours for adults and children 12 and over, and says children under 12 should not use it unless directed by a doctor. This is a label position with a DailyMed publication date of 2026-02-09 (SPL version 13), not a safety threshold established by study. (Source 32)
• Studied: ASPREE gave 100 mg of enteric-coated aspirin daily to 9,525 adults aged 70 and over for a median 4.7 years. (Source 17)
• Studied: ARRIVE gave enteric-coated aspirin tablets of 100 mg once daily to 6,270 patients for a median 60 months. (Source 25)
• Studied: The colorectal cancer follow-up covered scheduled treatment with 75-300 mg daily for a mean of 6.0 years. (Source 6)
• Studied: Mason's crossover pharmacokinetic study used single 325 mg doses as an unbuffered tablet and as two effervescent buffered solutions in 12 volunteers. (Source 11)
• Studied: The upper gastrointestinal bleeding study classified regular use as at least every other day and split doses at 325 mg daily or less versus more than 325 mg daily. (Source 18)

A common belief, and what the research shows

The belief: Buffered aspirin is the gentle kind, so taking the buffered tablet protects your stomach on long-term aspirin.

What the research shows: The one study that tested this against actual bleeding events found the opposite of gentleness. Buffered aspirin's relative risk of major upper gastrointestinal bleeding was 3.1 at 325 mg daily or less, slightly higher than plain aspirin's 2.6, and 7.0 above 325 mg daily against plain aspirin's 5.8. The authors' own words were that "The assumption that these formulations are less harmful than plain aspirin may be mistaken." The buffer does change pharmacokinetics - about 25% less intact aspirin reaches the circulation from a buffered effervescent solution, and urine pH and salicylate clearance shift - but that is not the same as protecting the stomach. FDA's own monograph singles out buffered aspirin only to warn that sodium-containing buffered preparations should be avoided by people with heart or kidney failure because of their sodium load.

Questions and answers

What is it?

Buffered aspirin is plain aspirin (acetylsalicylic acid) mixed with antacid ingredients in the same tablet or sachet. FDA's over-the-counter monograph defines it as aspirin buffered with permitted antacid ingredients so that the finished product has at least 1.9 milliequivalents of acid-neutralising capacity for every 325 mg of aspirin. The active drug is identical to ordinary aspirin; only the buffer is added. (Source 33)

What does it do in the body?

Aspirin permanently acetylates the enzyme cyclo-oxygenase-1 in platelets, at serine-529. Because platelets cannot make new enzyme, one daily dose keeps thromboxane production suppressed for the platelet's whole life, which is how aspirin reduces clotting. The same enzyme blockade elsewhere removes prostaglandins that reduce pain and fever and that protect the stomach lining. The antacid buffer neutralises acid and speeds stomach emptying but does not change the enzyme blockade. (Source 1)

Is it good or bad for you?

It depends entirely on who is taking it. In people who have already had a heart attack or stroke, pooled trials show serious vascular events falling from 8.2% to 6.7% a year, which is a real and worthwhile benefit. In people with no vascular disease the benefit shrinks to about 0.38% in absolute terms (one event prevented per 265 people) while major bleeding rises by about 0.47% (one extra bleed per 210 people), so the two roughly cancel out. In healthy adults over 70, ASPREE found no cardiovascular benefit at all and 38% more major haemorrhage. In children and teenagers with a viral illness it should not be used at all because of Reye's syndrome. (Source 3)

How do you get more of it?

Buffered aspirin is sold over the counter as a 325 mg tablet, and FDA's monograph permits buffered aspirin as an over-the-counter analgesic active ingredient. The marketed label directs adults and children 12 and over to take 2 tablets every 4 hours, with a full glass of water, up to 12 tablets in 24 hours. Antiplatelet dosing is a separate matter set by a prescriber. Salicylate also reaches the body from willow bark supplements and from salicylate-containing foods and botanicals, which is why those add to the same total. None of this is advice to the reader. (Source 32)

If it is harmful, what reduces it?

In salicylate overdose, treatment is aimed at supporting vital functions, increasing salicylate elimination and correcting the acid-base disturbance. Activated charcoal is used within about three hours of ingestion, alkaline diuresis is promoted by monitoring plasma electrolytes and pH if kidney function is normal, and haemodialysis or peritoneal dialysis is used in severe or life-threatening cases. Alkalinising the urine speeds salicylate removal, which is the same mechanism by which a buffered aspirin's antacid load was shown to change salicylate renal clearance. (Source 34)

Why might someone be low in it or missing it?

This question does not apply in the nutrient sense: aspirin is a manufactured drug, not something the body makes or needs. People go without it either because nobody has indicated it or because they cannot take it. The label rules out people allergic to aspirin or any other pain reliever, and FDA's professional labelling contraindicates it in people with the asthma, rhinitis and nasal polyps syndrome and in children or teenagers with viral infections. People with severe kidney failure, severe liver insufficiency or active peptic ulcer disease are told to avoid it, and about 9% of adults with asthma react to it. (Source 35)

Which whole foods contain it or feed it?

No whole food contains buffered aspirin, which is a manufactured tablet. Salicylates themselves do occur in plants: salicylates extracted from white willow are the origin of the drug, and willow bark supplements still deliver them. A United States Pharmacopeia safety review calculated that 240 mg of salicin from willow bark could yield 113 mg of salicylic acid, compared with 62 mg from an 81 mg aspirin tablet, and noted that supplement products are not required to carry the warnings aspirin must carry. (Source 14)

What happens if you do not have it?

Nothing happens to someone who has never needed it; aspirin is not a nutrient. For someone already established on long-term low-dose aspirin, stopping was associated in a 601,527-person Swedish cohort with a 37% higher rate of cardiovascular events, about one extra event a year for every 74 people who stopped. That is an observational association, not proof that stopping causes the events. For people with no vascular disease, the primary-prevention trials found that not taking aspirin cost very little: ASPREE found no significant difference in cardiovascular disease between aspirin and placebo. (Source 24)

How can you test for it?

There is no routine test for whether you need aspirin. Two kinds of measurement exist. Blood salicylate level is used to judge how severe an overdose is, and plasma salicylate is also used as a target in high-dose anti-inflammatory treatment. Whether the antiplatelet effect has taken hold can be measured by serum thromboxane B2, platelet function tests and urinary thromboxane metabolites: in 24 healthy volunteers on 100 mg of enteric-coated aspirin, serum thromboxane B2 was suppressed by 99.0% on average. These are research and poisoning tools, not routine checks. (Source 34)

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