UTRECHT, THE NETHERLANDS. The Food and Drug Administration (FDA) in the USA has not approved the use of aspirin for the prevention of a first cardiovascular event (heart attack, stroke and cardiovascular death). Nevertheless, it is estimated that 50 million Americans now take a daily aspirin (acetylsalicylic acid) for the primary prevention of cardiovascular events. This translates into roughly 10 billion to 20 billion tablets consumed annually in the USA alone.
There is no evidence that daily aspirin consumption protects against a first ischemic stroke. As a matter of fact, there is evidence that it may do more harm than good in low-risk patients with atrial fibrillation. In a 2005 study of 871 low-risk AF patients Japanese researchers conclude that daily aspirin therapy (150-200 mg/day) in this group is neither effective nor safe. They actually observed more cardiovascular deaths, strokes and TIAs in the aspirin group than in the placebo group. In addition, fatal or major bleeding was found to be more frequent in the aspirin group than in the placebo group. Overall, the incidence of strokes, deaths and other adverse events was 42% greater in the aspirin group than in the placebo group. The trial was stopped early since the probability that aspirin would prove superior to placebo in stroke prevention, if it continued, was deemed to be vanishingly small.
There is also no evidence that aspirin therapy provides a net benefit in the prevention of a first heart attack. In 2003, five clinical trials designed to determine the benefits of aspirin therapy in the prevention of a first heart attack were reviewed in a study funded by Bayer, the manufacturer of aspirin. Two of the trials, the Physicians Health Study and the British Doctors Trials, involved a total of 27,210 healthy men aged 40-84 years. The participants were followed for a mean of 5 and 6 years respectively. The rate of nonfatal heart attack was 0.28% per year in the aspirin group and 0.40% per year in the placebo group; that is, an absolute risk reduction of 0.12%.
Considering that the risk of hemorrhagic stroke and fatal bleeding is about 0.2% per year, and that of major gastrointestinal bleeding is about 0.5% per year, it is clear that long-term aspirin therapy for the prevention of a first heart attack (primary prevention) is not appropriate. This is recognized in the FDA's 2003 decision not to approve aspirin for long-term use in the primary prevention of heart attacks.
Now a group of researchers from the University of Utrecht and Harvard Medical School reports that aspirin therapy is ineffective, or even harmful, for most women without a history of cardiovascular disease. Their study (Women's Health Study) included 27,939 initially healthy women who were randomized to receive either placebos or 100 mg of aspirin every second day. During 10 years of follow-up, 340 major cardiovascular events (heart attack, stroke and cardiovascular death) were observed in the placebo group (0.24%/year) as compared to 312 events (0.22%/year) in the aspirin-treated group. However, aspirin therapy was of no net benefit when taking into account its associated increased risk of major bleeding, in particular, gastrointestinal bleeding. Especially noteworthy was the finding that aspirin treatment of women with a 10% or greater 10-year risk for coronary heart disease, as advocated by most guidelines, was not associated with a net benefit.
Dorresteijn, JAN, et al. Aspirin for primary prevention of vascular events in women: individualized prediction of treatment effects. European Heart Journal, Vol. 32, 2011, pp. 2962-69
Editor's comment: The above study adds to the abundant evidence that aspirin is of no net benefit if taken on a regular basis in the hope of preventing a first cardiovascular event (heart attack, stroke and cardiovascular death). Patients who have already suffered a heart attack or ischemic stroke may, however, benefit from aspirin therapy. An obvious question is how much aspirin is required on a daily basis to achieve optimum protection?
One 300-mg dose of aspirin irreversibly destroys the ability of platelets to form the aggregates that are involved in thrombotic, ischemic stroke. The platelets recover their ability to aggregate at a rate of about 10% a day. Thus, a prophylactic regimen of a one-time, 325-mg dose (standard dosage) followed by a daily dose of 81 mg (baby aspirin) or even half a baby aspirin would provide the full beneficial effect of aspirin as far as prevention of secondary cardiovascular events is concerned. Recent data suggest that 100 mg of aspirin every other day is also effective in suppressing platelet function.