Western Hemisphere nations such as Mexico and Colombia are major sources of illicit drugs such as cocaine, heroin, methamphetamine, and marijuana. Precursor chemicals used in the production of illicit fentanyl and other dangerous synthetic drugs often originate in China but typically enter the United States through Canada and Mexico. U.S. agencies implementing the National Drug Control Strategy conduct several activities to disrupt the flow of illicit drugs and dismantle the organizations that control them (see fig.). In December 2016, Congress established the Western Hemisphere Drug Policy Commission to, among other things, evaluate the U.S.-funded counternarcotics programs in the Western Hemisphere. In this context, GAO was asked to review key issues related to U.S. counternarcotics efforts in the Western Hemisphere. This report examines (1) U.S. agencies’ spending for counternarcotic efforts in the Western Hemisphere during fiscal years 2010- 2015, the most recent data available; (2) how agencies are gathering and sharing best practices and lessons learned from their counternarcotics efforts domestically and internationally; and (3) mechanisms U.S. agencies have used to address changing drug threats. GAO is not making any recommendations in this report.