Opioids are a class of drugs that include heroin and prescription pain relievers that produce analgesia and euphoria. More than 2 million Americans have an opioid use disorder. Acute effects include analgesia, respiratory depression, miosis, and euphoria. Overdose is a serious complication of opioid use, characterized by depressed level of consciousness and respiratory depression. It can be treated with naloxone. Withdrawal symptoms include dysphoria, yawning, tearing, diarrhea, cramps, nausea, and piloerection. Buprenorphine, methadone, clonidine, and lofexidine can be used to ameliorate the symptoms of withdrawal. However, supervised withdrawal alone rarely leads to long-term abstinence. There are a number of psychosocial treatments, including self-help groups, outpatient therapy, and residential treatment; the data on their effectiveness are limited. Pharmacotherapy with an opioid agonist (methadone or buprenorphine) is the most effective treatment. Long-acting injectable naltrexone, an opioid antagonist, is also effective, but it is more difficult to initiate.