Antimicrobial drugs are fed to hogs at sub-therapeutic levels to prevent disease and promote growth. However, there is concern that the presence of antimicrobial drugs in hog feed is a factor promoting the development of antimicrobial drug-resistant bacteria. This study uses a sample-selection model to examine the impact that use has on the productivity of U.S. hog operations. The analysis did not find a relationship between productivity and sub-therapeutic antibiotics fed during finishing, but productivity was significantly improved when fed to nursery pigs. Restrictions on feeding antimicrobial drugs during the nursery phase would likely impose significant economic costs on U.S. hog producers.