The trial use of aspirin therapy was based on clinical observation and on aspirin's possible mechanisms of action. Previous observations of diabetic patients who were taking large doses of aspirin for rheumatoid arthritis showed that the prevalence of retinopathy in this group was lower than the prevalence that would be expected in the diabetic population at large. Evidence suggested that diabetic patients have altered platelet aggregation and disaggregation, which may contribute to the capillary closure seen in retinopathy. This abnormality is reversed by aspirin in vitro . However, because of aspirin's other possible mechanisms of action and its well-known side effects, such as allergic, idiosyncratic, and intolerance reactions, the use of this therapy in the ETDRS was carefully controlled and monitored.