Naltrexone is an opioid antagonist, a drug which works by blocking the action of heroin, morphine and other similar opium-derived drugs at opioid receptors in the brain. Naltrexone, sold under the brand name Revia, is one of many treatments for opiate addiction, with methadone maintenance therapy being a popular alternative. It is also sometimes prescribed for alcohol addiction for its ability to reduce physical craving.
Naltrexone is generally given to heroin addicts as an aid to maintaining abstinence. This drug is intended to work by the much the same principle that Antabuse does, in that instead of directly medicating the addiction itself, it takes away the reward of using the drugs. Naltrexone makes it impossible to get "high" from opioid drugs like heroin and morphine, and therefore can help addicts deal with episodes of craving. (Antabuse works by interfering with the metabolism of alcohol, so that alcoholics who drink while taking Antabuse will become violently ill.) The problem with prescribing the drug for this person is that it relies on patient compliance, since all the individual needs to do to resume drug use is to discontinue taking naltrexone.
One of the problems with naltrexone patients who do relapse is that often times they will overdose, because their tolerance to their drug of choice will have dissipated over their period of abstinence. Also, naltrexone is usually not administered to an active addict, because it will cause sudden, severe withdrawal symptoms.
Naloxone, a cousin of naltrexone, works by the same pharmacological method of action and is commonly used to resuscitate opiate users who have overdosed. (By displacing the drug at the receptor sites, naloxone stops these drugs from causing respiratory suppression, which is the side effect in opiate overdose that causes death.)