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Primary prevention with aspirin

Systematic review

A new review examines randomised trials of aspirin for at least one year to prevent cardiovascular disease events in patients without a history of cardiovascular disease [1].

Systematic review

There were two criteria, randomised trials of aspirin chemoprevention in patients without known cardiovascular disease, and systematic reviews, trials and observational studies examining haemorrhagic stroke and gastrointestinal bleeding secondary to aspirin use. The aim was to examine the balance of benefit and harm for aspirin in patients with different levels of risk for heart disease.


Included were five randomised trials of aspirin on prevention of coronary heart disease, and nine articles on gastrointestinal bleeding or haemorrhagic stroke.

The five randomised trials had over 53,000 participants. The daily dose of aspirin was 500 mg in about 10% of these, and was 75 mg per day up to 325 mg every other day in most. Control was placebo or no treatment. Duration was 3.6 to 6.8 years. Most (80%) participants were middle-aged men.

Over about five years with control 2.4% had a coronary heart disease event, 0.6% died of a coronary heart disease event, 1.3% had a fatal or non-fatal stroke, and the all-cause mortality was 3.4%. Aspirin significantly reduced the number of coronary heart disease events (Figure 1) with a relative risk of 0.72 (95% CI 0.64 to 0.80). No other outcome was statistically different with aspirin. The number needed to treat with low dose aspirin for five years to prevent one coronary heart disease event that would not have happened with control was 190 (130 to 380).

Figure 1: CHD events in primary prevention trials

Estimates for haemorrhagic stroke and gastrointestinal bleeding were taken from meta-analyses. The net impact of the benefits and harms of aspirin prophylaxis is shown in Table 1 for three different levels of five-year risk. At 3% and higher the benefits begin to outweigh the possible harm.

Table 1: Net impact of benefits and harms of aspirin prophylaxis at different levels of risk


Estimated 5-year risk for CHD events at baseline





Effect on all-cause mortality No change No change No change
CHD events avoided (per 1,000) 3 8 14
Ischaemic strokes avoided (per 1,000) 0 0 0
Haemorrhagic stokes caused (per 1,000) 1 1 1
Major gastrointestinal bleeding events (per 1,000) 3 3 3


Previously it has been suggested that benefits of low dose aspirin outweigh the risks when the annual risk of a coronary event is 1%. This new analysis bears that out. The caution is that while coronary events are reduced, coronary death, strokes, and death from all causes were all unchanged by treatment.

Primary prevention and secondary prevention with aspirin are just different parts of a spectrum, as the similarity between Figure 1, and a L'Abbé plot of death from secondary prevention shows (Figure 2).

Figure 2: Deaths in secondary prevention trials for comparison


  1. M Hayden et al. Aspirin for the primary prevention of cardiovascular events: a summary of the evidence for the US preventive services task force. Annals of Internal Medicine 2002 136: 161-172.
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