An apple a day: Habit formation in medication adherence.

In this paper I study the causal effect of prescription duration on medical nonadherence and explore its effects on habit formation. By exploiting a recent change in policy made by Mexico’s Social Security Institute (IMSS), I analyze how patients’ prescription filling behavior changes in response to receiving 90-day prescriptions as opposed to 30-day prescriptions. Using a novel database from IMSS pharmacies’ administrative records which allows me to follow over four million hypertensive patients for a period of more than two years, I find that giving longer prescriptions improves medication adherence by significantly reducing the number of days in which patients are out of hypertensive medication. My analysis presents suggestive evidence that the mechanism through which longer prescriptions improve treatment compliance is by reducing the nonmonetary costs associated with therapeutic treatment. Additionally, I find evidence supporting the hypothesis that longer duration promotes good habit formation and may thus have a more permanent effect on patients’ adherence.