Policy makers around the world seek to encourage generic substitution. In this paper, the importance of prescribing physicians' imperfect agency is tested using the fact that some Swiss jurisdictions allow physicians to dispense drugs on their own account (physician dispensing, PD) while others disallow it. We estimate a model of physician drug choice with the help of drug claim data, finding a significant positive association between PD and the use of generics. While this points to imperfect agency, generics are prescribed more often to patients with high copayments or low incomes.