The US supply of generic drugs is heavily dependent on the global supply chain for sources of generic active pharmaceutical ingredients (APIs) for the US pharmaceutical market. Data from Clarivate Analytics' Cortellis Generics Intelligence database were analyzed to perform a systematic examination of generic APIs produced globally for the US market during 2020-21. We identified a total of 565 facilities producing 1,379 unique generic APIs across forty-two countries. India, China, and Italy were the top producers; 14 percent of APIs were manufactured in the US. About a third of APIs were manufactured by a single facility, and another third were manufactured by two or three facilities. More than one in every five APIs reflected markets in which current Food and Drug Administration standards would have failed to detect low competition because there were three or fewer API manufacturers despite there being four or more manufacturers of finished generic drugs. Monitoring the API supply is crucial to identifying vulnerabilities in the US pharmaceutical supply chain and identifying drugs that could represent potential priorities for domestic production. Incentives in the US may be needed to support API production to safeguard against supply-chain disruptions.