Ans. a (Naltrexone) (Ref KDT 6th/464, 467)Naltrexone differs from naloxone in being orally active and having a long duration of action (1-2 days) which makes it suitable for 'opioid blockade' therapy of postaddicts.Opioids & Opioid AntagonistsAgonist-antagonists (k-analgesics)Partial/weak m agonist + K antagonistPure antagonistNalorphineBuprenorphineNaloxonePentazocine NaltrexoneButorphanol NalmefeneNALTREXONE# Pure Opioid Antagonist like naloxone# Chemically related to naloaxone# More potent than naloxone.# Differs from naloxone in that it is orally active and has longer duration of action (1-2 days).# USES:- Hence suitable for "opioid blockade therapy" of postaddicts: 50 mg/day orally.- Also useful to reduce alcohol craving- Used to prevent relapse of habit drinking.# S/E:- Nausea- Headache- Heptotoxicity (at high doses).THE OPIOID ANTAGONISTSIntroduction# The pure opioid antagonist drugs naloxone, naltrexone, and nalmefene.# These agents have a relatively high affinity for m opioid binding sites.# They have lower affinity for the other receptors but can also reverse agonists at d and k sites.Pharmacokinetics# Naloxone is usually given by injection and has short duration of action (1-2 hours) when given by this route.# Nalmefene, the newest of these agents, is a derivative of naltrexone but is available only for intravenous administration.# Like naloxone, nalmefene is used for opioid overdose but has a longer half-life (8-10 hours).# When given intravenously to a morphine-treated subject, the antagonist completely and dramatically reverses the opioid effects within 1-3 minutes.Clinical Use# Naloxone is a pure antagonist and is preferred over older weak agonist-antagonist agents that had been used primarily as antagonists, e.g., nalorphine and levallorphan.# NALOXONE in the treatment of acute opioid overdose.# Because of its long duration of action, NALTREXONE has been proposed as a maintenance drug for addicts in treatment programs. A single dose given on alternate days blocks virtually all of the effects of a dose of heroin.# NALTREXONE decreases the craving for alcohol in chronic alcoholics, and it is FDA approved for this purpose.