ABSTRACT
Antagonists have zero efficacy and as such are very effective blockers of agonists. Their main limitations are that they can precipitate withdrawal in physically dependent drug users, and because they do not provide any reinforcement, there is little incentive for those dependent on illicit drugs to be compliant. Naltrexone is a long-acting, orally effective competitive antagonist at opioid receptors. It displaces any agonist present at the receptors and blocks the effects of any opioid subsequently administered. Naltrexone is predominantly metabolized by the liver and its major metabolite, 6-beta-naltrexol, is also an active opioid antagonist. The opiate-blocking effects of naltrexone last between 48 and 72 h. Naltrexone has not been shown to produce tolerance and therefore does not lead to physical dependence.
