In Summary, for those who aspirin daily for a year:

Benefits in NNT

  • 99.94% saw no benefit
  • 0% were helped by avoiding death
  • 0.05% were helped by preventing a non-fatal heart attack
  • 0.01% were helped by preventing a non-fatal stroke
  • 1 in 1667 were helped (cardiovascular problem prevented)
  • None were helped (prevented death)
  • 1 in 2000 were helped (prevented non-fatal heart attack)
  • 1 in 3000 were helped (prevented non-fatal stroke)

Harms in NNH

  • 0.03% were harmed by developing a major bleeding event*
  • 1 in 3333 were harmed (major bleeding event*)

*Required hospital admission and transfusion

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Source: Antithrombotic Trialists Collaboration. Aspirin in the primary and secondary prevention of vascular disease: collaborative meta-analysis of individual participant data from randomised trials. Lancet. 2009; 373(9678); 1849-60

Who Was in the Studies: Adults, middle age and older with at least one risk factor for heart disease, such as high blood pressure. The rate of stroke, heart attack, or death in this group was roughly 6% over 10 years.

Efficacy Endpoints: Heart attack, stroke, death

Harm Endpoints: Bleeding, death

Narrative: Aspirin blocks the action of platelets, reducing clots and ostensibly lowering the risk of heart attacks, strokes, and deaths. This review examined and summarized the magnitude of benefits from daily aspirin when compared to placebo for 'primary prevention', i.e. among patients who have never had a heart attack or stroke.

Aspirin did reduce certain clotting events (all of them nonfatal) but it also increased bleeding events. In the end the miniscule potential benefit does not seem worth it in comparison to the harms and in light of the aggregate impact.

Caveats: Notably, the numbers provided here are annual event-rates, suggesting these numbers might be expected for each single year of taking a daily aspirin. This implies a potential linear increase in both benefits and harms. More recent data from a large high quality trial in Japan, however, do not bear this out. In the Japanese Primary Prevention Project study of more than 14000 patients investigators halted the study after 5 years because benefits were not apparent while bleeding problems were.1 These data further support the interpretation that daily aspirin is not beneficial overall for primary prevention. They also support an unusual statement released by the United States Food and Drug Administration in mid-2014 that aspirin is not effective in primary prevention.2 This followed the FDA's review and denial of a request by Bayer to change aspirin's label to support an indication for primary prevention.

The apparent failure of aspirin to be helpful in this population highlights an important fact about medical treatment and the results of research on medical treatments: the more likely that patients in a study will have an event (a heart attack or a stroke, etc.) the more likely it is that they can potentially benefit from an effective intervention. Conversely in a group of healthy patients who are unlikely to have a heart attack or stroke, it is very difficult for a drug to successfully reduce heart attacks or strokes. This is intuitive: if there are no heart attacks in a group, it is not mathematically possible to reduce heart attacks. If there are very few heart attacks, it is mathematically difficult to reduce heart attacks. The more heart attacks in a group, the more room for improvement. Thus drugs that are effective in preventing events like heart attacks are always increasingly effective as the risk of heart attacks in the group increases.

Aspirin works to reduce events among patients who have a higher likelihood of having an event, and there remains an argument for aspirin in patients who are at exceptionally high risk but who have not yet had a heart attack or a stroke. This has led to some controversy in this issue, and varying interpretations. However the potential for aspirin to be beneficial in this group remains largely speculative, and the source article cited above examined even the highest risk patients who had not had a heart attack or stroke in trials, and there was no demonstrable benefit to aspirin even for these patients (though this subgroup was small in these trials). Thus we classify aspirin as 'Red' in this group based on the best available current data, which are extensive.

Author: David Newman, MD

Published/Updated: January 8, 2015

  1. The Title Bar

    The title bar is color-coded with our overall recommendation.

    • Green: Benefits outweigh risks.
    • Yellow: Unclear risk/benefit profile.
    • Red: Benefits do not outweigh risks.
    • Black: Obvious harms, no clear benefits.
  2. Tip content...