<prompt> Opioid antagonist that can be given orally?NaltrexoneNalaxoneNalbuophenePentazocin  <response>: Ans. a Naltrexone Ref KDT 6th464, 467Naltrexone 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-analgesicsPartialweak 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 mgday orally.- Also useful to reduce alcohol craving- Used to prevent relapse of habit drinking. SE:- 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. 