Insurance for prescription drugs is characterized by two types of cost-sharing: flat copayments and variable coinsurance. We develop a theoretical model to show that refill purchases of drugs are lower under coinsurance due to the consumer’s exposure to variation in drug prices. We test this empirically using claims data from eight large firms. Propensity score methods are used to create matched samples for the two insurance regimes. We find that when coinsurance and copayments have the same expected out-of-pocket of $9, at least 34% of patients under copayments would fully refill their medication over 90 days, compared to only 24% under coinsurance.