3. Old stuff
          3.1. Old pharm stuff (pre 2009)
              3.1.3. Pharmacology
                  3.1.3.5. Opioids
                      3.1.3.5.4. Opioid agonist-antagonists
 3.1.3.5.4.1. Pentazocine 

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