Opioid antagonist

[Ref: SH(4)2:p114]

Naloxone

Nonselective antagonist
* Acts on all 3 opioid receptors

Pharmacokinetics

Short duration of action = 30-45 minutes
--> May require repeated doses

Clinical uses

• Treatment of opioid-induced depression of ventilation
  * Post-operatively
  * In neonates due to maternal use of opioid
• Treatment of opioid overdose
• Detection of suspected opioid physical dependence

Dosage

1-4 microgram/kg IV
--> Reverses opioid-induced analgesia and depression of ventilation

Neuraxial opioid

Continuous infusion of 5 microgram/kg/hour IV
--> Prevents depression of ventilation without altering analgesia due to neuraxial opioid

Side effect

• Reversal of opioid-induced analgesia
• Antagonism of inhaled anaesthetics
  * Possible drug-induced activation of cholinergic arousal system in the brain, independent of opioid receptors
• Nausea and vomiting
• CVS stimulation
  * Tachycardia
  * Hypertension
  * Pulmonary oedema
  * Cardiac dysrhythmia (VF)
• Opioid withdrawal in neonates of opioid dependent mothers

Naltrexone

Highly effective orally
* Unlike naloxone (which is short-acting)
* Sustained antagonism for up to 24 hours

Used to treat addicts

Nalmefene

Pure opioid antagonist

Equipotent to naloxone

Longer duration than naloxone
--> Better protection for ventilatory depression

Opioid agonist-antagonists

[Ref: SH(H)2:p111]

Opioid agonist-antagonists bind to mu receptors
--> Limited response or no effect

Side effect profile is similar to opioid agonist, except:

• May cause dyphoric reactions
• Limited depression of ventilation
• Low potential to produce physical dependency