This paper examines the effect of direct-to-consumer advertising (DTCA) on doctor choice of prescription drugs. Using antihistamines as an example, we show that DTCA has a small and insignificant effect on the choice of brand despite the massive DTCA expenditure incurred in this class. In contrast, direct-to-physician advertising (i.e., detailing and medical journal advertising) has a larger and long-lasting effect on prescription choice.