3. Old stuff
          3.1. Old pharm stuff (pre 2009)
              3.1.3. Pharmacology
                  3.1.3.5. Opioids
                      3.1.3.5.3. Opioid agonists
                          3.1.3.5.3.9. Other opioid agonists
 3.1.3.5.3.9.1. Codeine 

Codeine

[SH4:p115]

Structure

  • Substitution of a methyl group for the hydroxyl group on carbon 3 of morphine

Structure-activitiy relationship

  • Methyl group substitution
    --> Limits first-pass hepatic metabolism
    --> Improved efficacy with oral administration

NB:

[MCQ:Q148] Codeine has less first pass effect than morphine

Pharmacodynamics

  • Less liable for physical dependence than morphine
  • Produces minimal sedation, N&V, and constipation
  • Even in large dose, apnoea is unlikely.

Pharmacokinetics

Elimination half-time = 3 - 3.5 hours

Metabolism

  • About 10% of codeine is demethylated in liver
    --> Becomes morphine
    --> Accounts for analgesic effect
  • The remaining codeine is demethylated to
    --> Norcodeine
  • Norcodeine is inactive
    --> Excreted by kidney, or conjugated

Clinical

Administration

Not recommended for IV administration
* Due to histamine-induced hypotension

Antitussive

Codeine 15 mg PO

Analgesia

Maximal analgesia occurs with 60mg codeine
* Equivalent to 650mg aspirin