Pentazocine

[SH4:p118]

Usage

• Treatment of chronic pain where there is a high risk of physical dependence
• Relief of moderate pain

Structure

• Benzomorphan derviative
• Opioid agonist action as well as weak antagonist actions

Pharmacodynamics

Potency

• About 1/5 as potent as an antagonist as nalorphine
• Pentazocine 50mg PO has equivalent analgesic potency as 60mg of codeine
• Pentazocine 20-30mg IM produces analgesia, sedation, and respiratory depression similar to 10mg of morphine

Mechanisms of actions

• Antagonist at MOP receptors
• Agonist at DOP and KOP receptors
  --> Can still be antagonised by naloxone

Side effects

Most common side effects of pentazocine are

• Sedation
  * Due to activation of KOP receptors
• Diaphoresis
• Dizziness

Other side effects

• Antagonist action at MOP receptors is sufficient to precipitate withdrawal symptoms
• N&V is less common than with morphine
• Dysphoria (including fear of impending death) is associated with high doses of pentazocine
• Pentazocine can increase plasma concentration of catecholamines
• Respiratory depression

NB:

• Pentazocine does NOT produce miosis

Pharmacokinetics

Absorption

• Well absorbed after oral or parenteral administration
• Extensive first-pass hepatic metabolism
  --> Oral bioavailability = 20%

Metabolism

• Oxidation of terminal methyl groups
  --> Inactive glucuronide conjugates (excreted in urine)

Elimination

• About 5-25% of pentazocine is excreted unchanged in the urine
• <2% excreted in bile
• Elimination half-time = 2-3 hours

Clinical

Administration

10-30mg IV or 50mg PO