This medication is used to prevent people who have been addicted to certain drugs (opiates) from taking them again. It is used as part of a complete treatment program for drug abuse (e.g., compliance monitoring, counseling, behavioral contract, lifestyle changes). This medication must not be used in people currently taking opiates, including methadone. Doing so can cause sudden withdrawal symptoms.

Naltrexone belongs to a class of drugs known as opiate antagonists.