Opioid use disorder (OUD) is a significant health problem in the United States and many other countries. A combination of issues, most notably increased prescription of opioid analgesics, has resulted in climbing rates of opioid abuse and overdose over the last decade. This ongoing epidemic has produced a growing population of patients requiring treatment for OUD. Medications such as methadone and buprenorphine have well documented success rates in treating the disorder compared with placebo. However, significant percentages of the population still fail to maintain abstinence or reduce illicit opioid use while using such medications. Genetic variation may play a role in this variability in outcome through pharmacokinetic or pharmacodynamic effects on OUD medications, or by affecting the rate of negative side effects and adverse events. This review focuses on the existing literature on the pharmacogenetics of OUD treatment, with specific focus on medication metabolism, treatment outcomes, and adverse events.