There is wide dispersion in pharmaceutical prices across countries with comparable quality standards. Under monopoly, off-patent and generic drug prices are at least four times higher in the United States than in comparable Englishspeaking high income countries. With five or more competitors, off-patent drug prices are similar or lower. Our analysis shows that differential US markups are largely driven by the market power of drug suppliers and not due to wholesale intermediaries or pharmacies. Furthermore, we show that the traditional mechanism of reducing market power – free entry – is limited because implied entry costs are substantially higher in the US.