Background: Generic prescription drugs made by different manufacturers may vary in color or shape, and switching among these drug products may interrupt medication use.Objective: To determine whether nonpersistent use of generic drugs among patients with cardiovascular disease after myocardial infarction (MI) is associated with inconsistent appearance of their medications. Design: Cohort and nested case–control studies.Setting: Claims from a commercial health insurance database in the United States. Patients: Patients discharged after hospitalization for MI between 2006 and 2011 who initiated treatment with a generic β-blocker, angiotensin-converting enzyme inhibitor, angiotensin II–receptor blocker, or statin. Case patients discontinued their index medication for at least 1 month; control patients continued treatment. Control patients were matched to case patients on therapeutic class, number of dispensings before nonpersistence, sex, and age. Measurements: Rates of changes in pill color and shape during the year after MI were calculated. Next, 2 refills preceding nonpersistence were evaluated to determine whether pill color or shape had changed. Odds of discordance among case and control patients were compared using conditional logistic regression. Results: A total of 29% of patients (3286 of 11 513) had a change in pill shape or color during the study. Statins had the most changes in appearance, whereas β-blockers had the fewest. A total of 4573 episodes of nonpersistence was matched to 19 881 control episodes. The odds of nonpersistence in case patients increased by 34% after a change in pill color (adjusted odds ratio, 1.34 [95% CI, 1.12 to 1.59]) and 66% after a change in pill shape (adjusted odds ratio, 1.66 [CI, 1.43 to 1.94]). Limitation: Only 3 categories of drugs indicated after MI were evaluated, and clinical outcomes were not addressed. Conclusion: Variation in the appearance of generic pills is associated with nonpersistent use of these essential drugs after MI among patients with cardiovascular disease. Primary Funding Source: Agency for Healthcare Research and Quality and the Harvard Program in Therapeutic Science.