Naltrexone (brand name ReVia) is an OPIOID ANTAGONIST (i.e., a blocker of substances with morphine-like actions), with a structure very similar to another antagonist, NALOXONE. It also closely resembles the potent ANALGESIC (painkiller) OXYMORPHONE. The differences between naloxone and naltrexone are restricted to a simple substitution on the nitrogen atom, with naltrexone having a methylcyclopropyl group, yet this small substitution changes the pharmacology of the compound dramatically. Naltrexone has no analgesic actions by itself and has the ability to antagonize, or reverse, virtually all the effects of morphine-like drugs. Like naloxone, naltrexone will precipitate WITHDRAWAL in physically dependent people.

Naltrexone is rapidly metabolized in the liver, but one of its metabolites is 6-naltrexol, which has some activity and a longer duration of action. In the 1990s, naltrexone was used to treat opiate addiction and for rapid opioid detoxification. Its greater potency than naloxone, along with its greater...