The United States Food and Drug Administration (US FDA) is a department of health and human services federal agency. If the FDA notices major violations of federal regulations, it sends warning letters to pharmaceutical makers, distributors, and clinical investigators. The nature of the infractions discovered, as well as the FDA's proposed corrective activities, are detailed in these warning letters. Following the completion of the corrective activities, the recipient may request a follow-up inspection. This review article examines the current state of warning letters issued by the US Food and Drug Administration (FDA) from 2019 to 2021. The main goal of this study is to examine warning letters related to drug and determine what the significant violations.