Larson RJ and Fisher ES (2004) Should aspirin be continued in patients started on warfarin? J Gen Intern Med 19: 879–886

There is a lack of consensus on whether patients starting warfarin therapy should continue taking daily aspirin. Although antithrombotic protection is likely to be superior when both agents are used, some clinicians fear that the risk of increased bleeding is unacceptably high. In an attempt to resolve this problem, Larson and Fisher have carried out a systematic review and meta-analysis of nine randomized trials comparing warfarin plus aspirin versus warfarin alone.

There were five trials covering mechanical heart valves, of which four used the same INR in both treatment groups. The pooled results of these four studies indicated that, compared to warfarin alone, the combination of warfarin and aspirin significantly decreased the rates of thromboembolic events and all-cause mortality, while increasing major bleeding. The fifth mechanical heart valve trial, in which a reduced INR was used in the warfarin plus aspirin group, showed no difference in the rate of thromboembolic events but a decrease in major bleeding and all-cause mortality with combination therapy. Three trials covering secondary prevention of myocardial infarction (MI) showed no significant differences in subsequent MI, major bleeding or all-cause mortality between the treatment groups. The single trial covering atrial fibrillation was terminated early and conclusions could not be drawn from the results.

In conclusion, Larson and Fisher's analysis suggests that combination therapy with warfarin plus aspirin is appropriate for mechanical heart valve patients, whereas there are insufficient data to guide this decision in other common warfarin indications.