In this work we investigate the causal impact of cost sharing schemes on drug compliance using a Difference-in-Differences approach within a quantile regression framework. We exploit a series of natural experiments occurred in Italy between 2000 and 2010, referring to the introduction of regional co-payment schemes. We find that co-payments have a negative impact on compliance, causing a reallocation of individuals from the upper to the lower tail of the compliance distribution, thus hurting the already narrow group of good compliers. The empirical evidence suggests also that gender, clinical history and geographic residence are important drivers of compliance, particularly within the group of poor compliers.