Drug addiction is the most severe form of a substance use disorder (SUD). An SUD develops when a person’s continued use of alcohol and/or drugs causes significant issues, such as health problems, disability, and failure to meet responsibilities at work, school, or home. An SUD can range from mild to severe. Addiction is a complex, chronic brain disease characterized by drug craving, seeking, and use that persists even in the face of devastating life consequences. Addiction results largely from brain changes that stem from prolonged drug use—changes that involve multiple brain circuits, including those responsible for governing self-control and other behaviors. Drug addiction is treatable, with medications (for some addictions) and/or behavioral therapies. However, relapse is common and can happen even after long periods of abstinence, underscoring the need for long-term support and care. Relapse does not signify treatment failure, but rather should prompt treatment re-engagement or modification.