Aspirin

Safety and precautions

All nonsteroidal anti-inflammatory drugs (NSAIDs) including aspirin can have serious, even life-threatening side effects. Use them only under medical supervision.

Modest evidencesignificant effects in at least three small but well-designed randomized controlled trials (RCTs), or one or more well-designed, mid-sized clinical studies of reasonably good quality (RCTs or observational studies), or several small studies aggregated into a meta-analysis (this is the CancerChoices definition; other researchers and studies may define this differently) of substantially increased all-cause mortalitydeath, or the death rate in a population; cancer studies often report all-cause mortality (death from any cause) and cancer-specific mortality (death due to the cancer under investigation) among people with postmenopausal hormone receptor-positive breast cancer using low-dose aspirin (81 mg or less)

• Substantially worse (67% higher) all-cause mortality among people with postmenopausal hormone receptor-positive breast cancer using low-dose aspirin (81 mg or less) compared to no use in a large observational study¹Strasser-Weippl K, Higgins MJ et al. Effects of celecoxib and low-dose aspirin on outcomes in adjuvant aromatase inhibitor-treated patients: CCTG MA.27. Journal of the National Cancer Institute. 2018 Sep 1;110(9):1003-1008.

Extreme caution is advised for people with low body weight, as shown in a pooled analysis of 10 very large RCTsrandomized controlled trial, a study design in which people are randomly assigned to either an experimental group or a control group to compare the outcomes from different treatments; an RCT is considered a strong design for determining a therapy’s effects:²Rothwell PM, Cook NR et al. Effects of aspirin on risks of vascular events and cancer according to bodyweight and dose: analysis of individual patient data from randomised trials. Lancet. 2018 Aug 4;392(10145):387-399. 

• Higher risk of sudden death among people using aspirin with low body weight for the dose received 
• 52% higher all-cause mortality among people weighing less than 50 kg (110 pounds) receiving 75–100 mg of aspirin
• 20% higher 3-year risk of cancer among people aged 70 years or older using aspirin, especially those weighing less than 70 kg (154 pounds)

Cancer mortality

Similar results may be in play for mortality after a cancer diagnosis. One study found a trend toward higher risk of cancer progression among elderly patients with stage 3 or 4 cancers treated with aspirin.³McNeil JJ, Gibbs P et al. Effect of aspirin on cancer incidence and mortality in older adults. Journal of the National Cancer Institute. 2020;djaa114.

Cancer risk or mortality

Use of aspirin is associated with higher risk or mortality regarding some types of cancer, among specific groups, or with specific doses.

Good evidencesignificant effects in one large or several mid-sized and well-designed clinical studies (randomized controlled trials (RCTs) with an appropriate placebo or other strong comparison control or observational studies that control for confounds) (this is the CancerChoices definition; other researchers and studies may define this differently) of moderately higher cancer mortality among healthy older adults treated with aspirin; aspirin may not be a wise choice for cancer prevention among older adults. 

• Healthy older adults treated with aspirin experienced 31% higher cancer mortality during a median of 4.7 years of follow-up in a very large RCTrandomized controlled trial, a study design in which people are randomly assigned to either an experimental group or a control group to compare the outcomes from different treatments; an RCT is considered a strong design for determining a therapy’s effects.⁴McNeil JJ, Nelson MR et al; ASPREE Investigator Group. Effect of aspirin on all-cause mortality in the healthy elderly. New England Journal of Medicine. 2018 Oct 18;379(16):1519-1528.

Modest evidencesignificant effects in at least three small but well-designed randomized controlled trials (RCTs), or one or more well-designed, mid-sized clinical studies of reasonably good quality (RCTs or observational studies), or several small studies aggregated into a meta-analysis (this is the CancerChoices definition; other researchers and studies may define this differently) of higher risk of recurrent advanced adenomas and/or 2 or more adenomas after colorectal adenoma surgery among people using aspirin 4 or more days/week and using vitamin D₃

• Higher risk of recurrent advanced adenomas and/or 2 or more adenomas after colorectal adenoma surgery among people using aspirin or other nonsteroidal anti-inflammatory drugs (NSAIDs) 4 or more days/week and using 1000 IU/day vitamin D₃ compared to those with less frequent NSAID use in a large RCT⁵Calderwood AH, Baron JA, et al. No evidence for posttreatment effects of vitamin D and calcium supplementation on risk of colorectal adenomas in a randomized trial. Cancer Prevention Research. 2019 May;12(5):295-304.

Modest evidence of moderately higher risk of lung cancer among women treated with 15 or more aspirin tablets per week

• 55% higher risk of lung cancer among women treated with 15 or more aspirin tablets per week compared with no aspirin, but no evidence of an effectoverall, one or more studies did not demonstrate that a treatment or intervention led to an expected outcome; this does not always mean that there is no effect in clinical practice, but that the studies may have been underpowered (too few participants) or poorly designed. Larger, well-designed studies provide more confidence in making assessments. on risk at lower doses in a very large observationala type of study in which individuals are observed or certain outcomes are measured, but no attempt is made to affect the outcome (for example, no treatment is given); an example is a study that records people’s diets, but doesn’t try to alter their diets, and looks for patterns of disease or other outcomes related to different foods study⁶Feskanich D, Bain C et al. Aspirin and lung cancer risk in a cohort study of women: dosage, duration and latency. British Journal of Cancer. 2007 Nov 5;97(9):1295-9.

Weak evidenceone or more case studies, supported by animal evidence OR small treatment effects of limited clinical significance OR studies with no controls OR weak trends of effects (this is the CancerChoices definition; other researchers and studies may define this differently) of higher colorectal cancer mortality after 5 to 10 years among people treated with aspirin; also see substantial evidence of lower colorectal cancer incidence and mortality among people treated with cancer in How can aspirin help you? What the research says ›

• A weak trendan apparent change due to a therapy, close to but not achieving full statistical significance (this is the CancerChoices definition; other researchers and studies may define this differently toward higher colorectal cancer mortality after 5 to 10 years among people treated with aspirin compared to controls in meta-analysisa statistical analysis that combines the results of two or more research studies; the results of smaller research studies addressing the same or similar questions can be analyzed as though they are one bigger, more powerful study of 2 RCTs⁷Guirguis-Blake JM, Evans CV, Perdue LA, Bean SI, Senger CA. Aspirin use to prevent cardiovascular disease and colorectal cancer: updated evidence report and systematic review for the US Preventive Services Task Force. JAMA. 2022 Apr 26;327(16):1585-1597.

Weak evidence of higher risk of Barrett’s esophagus among people with gastroesophageal reflux symptoms treated with aspirin; also see evidence of lower risk of Barrett’s esophagus among people treated with aspirin in How can aspirin help you? What the research says ›

• A weak trend toward higher risk of Barrett’s esophagus among people with gastroesophageal reflux symptoms treated with aspirin compared to no aspirin in a meta-analysis of 3 observational studies⁸Eusebi LH, Telese A et al. Systematic review with meta-analysis: risk factors for Barrett’s oesophagus in individuals with gastro-oesophageal reflux symptoms. Alimentary Pharmacology & Therapeutics. 2021 May;53(9):968-976. 

Side effects or adverse events

Aspirin can have many side effects, some potentially very serious. For this reason, we highly recommend having a physician monitor your use and guide you in looking for indications of any serious conditions.

Common side effects:⁹Aspirin Tablet Side Effects by Likelihood and Severity. Web MD. Viewed February 8, 2021; Cook NR, Lee IM, Zhang SM, Moorthy MV, Buring JE. Alternate-day, low-dose aspirin and cancer risk: long-term observational follow-up of a randomized trial. Annals of Internal Medicine. 2013 Jul 16;159(2):77-85.

Rare side effects:¹³Aspirin Tablet Side Effects by Likelihood and Severity. Web MD. Viewed February 8, 2021; McNeil JJ, Wolfe R et al, ASPREE Investigator Group. Effect of aspirin on cardiovascular events and bleeding in the healthy elderly. New England Journal of Medicine. 2018 Oct 18;379(16):1509-1518; McQuilten ZK, Thao LTP et al. Effect of low-dose aspirin versus placebo on incidence of anemia in the elderly : a secondary analysis of the aspirin in reducing events in the elderly trial. Annals of Internal Medicine. 2023 Jun 20.

Caution is advised for people with hypertension or risk factors for gastrointestinal bleeding. One review does not advise discontinuing use of aspirin and other NSAIDs among people with cancer, but the authors recommend careful monitoring for side effects such as cardiovascular events (with COX-2 inhibitors) or gastrointestinal bleeding.¹⁴Solheim TS, Fearon KC, Blum D, Kaasa S. Non-steroidal anti-inflammatory treatment in cancer cachexia: a systematic literature review. Acta Oncologica. 2013 Jan;52(1):6-17.

Do not use (contraindications)

Allergic reactions to aspirin can be fatal. If you suspect or know you are allergic to aspirin or salicylates, do not use aspirin.

Drug interactions

The US Food and Drug Administration cautions that some nonsteroidal anti-inflammatory drugs (NSAIDs), including those in over-the-counter products such as ibuprofen and naproxen, can interfere with the antiplatelet action of low-dose aspirin used for cardioprotection.¹⁵FDA Drug Safety Communication: FDA strengthens warning that non-aspirin nonsteroidal anti-inflammatory drugs (NSAIDs) can cause heart attacks or strokes. US Food and Drug Administration. February 2, 2015. Viewed October 21, 2021. 

Aspirin can also potentially interact with blood thinners, selective serotonin reuptake inhibitors (SSRIs), other nonsteroidal anti-inflammatory drugs, various antihypertensives, methotrexate, steroids, valproic acid, ginkgo biloba, and other drugs. Be sure to mention to your physician that you are taking any of these medications before using aspirin.