Naltrexone, sold under the brand names Revia and Vivitrol among others, is a medication primarily used to manage alcohol use or opioid use disorder by reducing cravings and feelings of euphoria associated with substance use disorder. It has also been found effective in the treatment of other addictions and may be used for them off-label. An opioid-dependent person should not receive naltrexone before detoxification. It is taken by mouth or by injection into a muscle. Effects begin within 30 minutes,though a decreased desire for opioids may take a few weeks to occur. Side effects may include trouble sleeping, anxiety, nausea, and headaches. In those still on opioids, opioid withdrawal may occur. Use is not recommended in people with liver failure. It is unclear if use is safe during pregnancy. Naltrexone is an opioid antagonist and works by blocking the effects of opioids, including both opioid drugs as well as opioids naturally produced in the brain. Naltrexone was first made in 1965 and was approved for medical use in the United States in 1984. Naltrexone, as naltrexone/bupropion (brand name Contrave), is also used to treat obesity. Medical uses Alcohol use disorder Naltrexone has been best studied as a treatment for alcoholism. Naltrexone has been shown to decrease the quantity and frequency of ethanol consumption. It does not appear to change the percentage of people drinking. Its overall benefit has been described as "modest". Acamprosate may work better than naltrexone for eliminating alcohol abuse, while naltrexone may decrease the desire for alcohol to a greater extent. The Sinclair method is a method involving opiate antagonists such as naltrexone to treat alcoholism. The person takes the medication once, about an hour before consuming alcohol, to curb the urge to drink. The opioid antagonist is thought to block the positive reinforcement effects of alcohol and may assist the person to stop or reduce their drinking. Opioid use Long-acting injectable naltrexone, under the brand name Vivitrol, is an opioid receptor antagonist, blocking the effects of heroin and other opioids, and decreases heroin use compared to a placebo. Unlike methadone and buprenorphine, it is not a controlled medication. It may decrease cravings for opioids after a number of weeks, and decreases the risk of overdose, at least during the time period that naltrexone is still active, though concern about risk of overdose for those stopping treatment remains. It is given once per month and has better