Opioid antagonists have no effect when administered alone. When given after a dose of agonist they promptly reverse all of the action of the agonist.
Mu receptor antagonist predominantly but affect all receptors.
Naloxone is the drug of choice for opioid overdose.
In addicts antagonist rapidly reverses the effect of agonists such as heroin & precipitate the symptoms of withdrawal
Alvimopam is modified analogue of Naloxone, given orally or parentally.
It selectively inhibits peripheral mu receptors, with minimum CNS penetration
Used in post op (bowel resection surgery problem of paralytic ileus). It is used because has no CNS action, only localized mu receptors. Selectively GIT peripheral receptors are blocked.
- Reverses the coma& respiratory depression of opioid overdose, within 5 minute of I/V administration respiratory depression is reversed, patients is revived.
- Competitive antagonist at µ, K,d receptors. High affinity for µ receptors
Methyl naltrexone bromide
Is effective orally and is long acting. It is a selective analog devoid of side effects, esp. constipation in terminally ill, seriously advanced diseases.
It is used in addicts’ maintenance programmes, decreasing craving for drug. Baseline beta endorphins increase also decreases alcohol craving.
Uses of Opioid Antagonists
- Opioid induced toxicity
- Diagnose physical dependence in those on opioids. If denial or not apparent, precipitation of severe withdrawal syndrome by giving.
- Treatment of compulsive opioid users
- Neonatal respiratory depression, to overcome if morphine, pethidine used. Although does not cause but may lead to respiratory depression.
- Patients of septic shock –naloxone is especially used where opioid peptides are released due to stress causing hypotension.