Nalmexone (INN; also known as nalmexone hydrochloride (USAN) or by the development codes EN-1620A and UM-592) is a semisynthetic, opioid partial agonist or mixed agonist-antagonist with both analgesic and narcotic antagonist properties that was never marketed. In clinical studies it was found to have comparable analgesic efficacy to morphine, though with several-fold reduced potency. In addition, nalmexone's side effects, the most common of which were sleepiness and sweating, were reported to be similar to those of morphine, albeit with a noticeably higher degree of incidence.

Nalmexone can be synthesized from oxymorphone: