Television advertising of prescription drugs is controversial, and it remains illegal in all but two countries. Much of the opposition stems from concerns that advertising directly to consumers may inefficiently distort prescribing patterns toward the advertised product. Despite the controversy surrounding the practice, its effects are not well understood. Exploiting a discontinuity in advertising along the borders of television markets, I estimate that television advertising of prescription antidepressants exhibits significant positive spillovers on rivals’ demand. I then construct and estimate a multi-stage demand model that allows advertising to be pure category expansion, pure business stealing, or some of each. Estimated parameters indicate advertising has strong market-level demand effects that tend to dominate business-stealing effects. Spillovers are both large and persistent. Using the demand estimates and a stylized supply model, I explore the consequences of the positive spillovers on firm advertising choice. Compared with a competitive benchmark in which firms optimally free ride, simulations suggest a category-wide cooperative advertising scenario would produce a significant increase in total advertising.