Aspirins effects on blood clotting (as an antiplatelet agent) were first noticed in 1950 by Lawrence Craven. Craven, a family doctor in California, had been directing tonsillectomy patients to chew Aspergum, an aspirin-laced chewing gum. He found that an unusual number of patients had to be hospitalized for severe bleeding, and that those patients had been using very high amounts of Aspergum. Craven began recommending daily aspirin to all his patients, and claimed that the patients who followed the aspirin regimen (about 8,000 people) had no signs of thrombosis. However, Cravens studies were not taken seriously by the medical community, because he had not done a placebo-controlled study and had published only in obscure journals. 
The idea of using aspirin to prevent clotting diseases (such as heart attacks and strokes) was revived in the 1960s, when medical researcher Harvey Weiss found that aspirin had an anti-adhesive effect on blood platelets (and unlike other potential antiplatelet drugs, aspirin had low toxicity). Medical Research Council haematologist John OBrien picked up on Weisss finding and, in 1963, began working with epidemiologist Peter Elwood on aspirins anti-thrombosis drug potential. Elwood began a large-scale trial of aspirin as a preventive drug for heart attacks. Nicholas Laboratories agreed to provide aspirin tablets, and Elwood enlisted heart attack survivors in a double-blind controlled study—heart attack survivors were statistically more likely to suffer a second attack, greatly reducing the number of patients necessary to reliably detect whether aspirin had an effect on heart attacks. The study began in February 1971, though the researchers soon had to break the double-blinding when a study by American epidemiologist Hershel Jick suggested that aspirin prevented heart attacks but suggested that the heart attacks were more deadly. Jick had found that fewer aspirin-takers were admitted to his hospital for heart attacks than non-aspirin-takers, and one possible explanation was that aspirin caused heart attack sufferers to die before reaching the hospital; Elwoods initial results ruled out that explanation. When the Elwood trial ended in 1973, it showed a modest but not statistically significant reduction in heart attacks among the group taking aspirin.
Several subsequent studies put aspirins effectiveness as a heart drug on firmer ground, but the evidence was not incontrovertible. However, in the mid-1980s, with the relatively new technique of meta-analysis, statistician Richard Peto convinced the U.S. FDA and much of the medical community that the aspirin studies, in aggregate, showed aspirins effectiveness with relative certainty. By the end of the 1980s, aspirin was widely used as a preventive drug for heart attacks and had regained its former position as the top-selling analgesic in the U.S.
In 2018, three major clinical trials cast doubt on that conventional wisdom, finding few benefits and consistent bleeding risks associated with daily aspirin use. Taken together, the findings led the American Heart Association and American College of Cardiology to change clinical practice guidelines in early 2019, recommending against the routine use of aspirin in people older than 70 years or people with increased bleeding risk who do not have existing cardiovascular disease.