Most countries regulate pharmaceutical prices, either directly or indirectly, on the assumption that competition is at best weak in this industry. This paper tests the hypothesis that regulation of manufacturer prices and retail pharmacy margins undermines price competition. We use data from seven countries for 1992 to examine price competition between generic competitors (different manufacturers of the same compound) and therapeutic substitutes (similar compounds) under different regulatory regimes. We find that price competition between generic competitors is significant in unregulated or less regulated markets (United States, United Kingdom, Canada, and Germany) but that regulation undermines generic competition in strict regulatory systems (France, Italy, and Japan). Regulation of retail pharmacy further constrains competition in France, Germany, and Italy. Regulation thus undermines the potential for significant savings on off‐patent drugs, which account for a large and growing share of drug expenditures. Evidence of competition between therapeutic substitutes is less conclusive owing to data limitations.