In Summary, for those who took the aspirin:

Benefits in NNT

  • 98% saw no benefit
  • 0.3% were helped by avoiding death
  • 1.3% were helped by preventing a non-fatal heart attack
  • 0.5% were helped by preventing a non-fatal stroke
  • 1 in 50 were helped (cardiovascular problem prevented)
  • 1 in 333 were helped (prevented death)
  • 1 in 77 were helped (prevented non-fatal heart attack)
  • 1 in 200 were helped (prevented non-fatal stroke)

Harms in NNT

  • 0.25% were harmed by developing a major bleeding event*
  • 1 in 400 were harmed (major bleeding event*)

*Required hospital admission and transfusion

View As:

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

Antithrombotic Trialists Collaboration. Collaborative meta-analysis of randomised trials of antiplatelet therapy for prevention of death, myocardial infarction, and stroke in high risk patients. BMJ. 2002 Jan 12;324(7329):71-86.

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 'secondary prevention', i.e. among patients who have had a recent heart attack or stroke.

Aspirin works: those taking aspirin in these studies suffered fewer heart attacks, strokes, and deaths than those taking a placebo, at the cost of a small number of bleeding events. In addition, the benefits outlined here were seen after just over two years of daily aspirin therapy, in contrast to the 4 and 5 year periods seen with many other cardiovascular preventive interventions.

Caveats: Aspirin can cause bleeding events and gastrointestinal problems (ulcers, indigestion, etc.). However the cost of generic aspirin is very low and the benefits are considerably more common than the harms, making aspirin an excellent intervention for the right patients. This intervention does not translate, however, to patients who are not at very high risk (i.e. those who have established disease). Note our related summary on the topic of aspirin for 'primary prevention'.

For those who cannot tolerate aspirin other antiplatelet agents such as clopidogrel, ticlopidine, and related drugs are probably very reasonable and effective alternatives and seem to have similar beneficial effects, despite having more side effects.

Author: David H. Newman, MD

Published/Updated: July 10, 2011

  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...