Indications:

• Weak opioids - mild to moderate pain
• Strong opioids - Severe pain

Examples:

Weak opioid analgesics - Codeine, Dihydrocoedeine

Strong opioid analgesics - Morphine, Diamorphone, Pethidine, Fentanyl

Partial/mixed agonist opioids - Buprenorphine, pentazocine

MOA:

• Mimic endogenous opioids - Act on opioid receptors (MOP, KOP, DOP, ORL1)
• Opioids decrease neuronal transmission:
  • Decrease opening of voltage dependent Ca channels
  • Increase K outflow
  • Decreases Ca release from intracellular stores
  • Hyperpolarisation = decreased exocytosis of transmitter vesicles

Sites of CNS opioid action:

• PAG - can dampen down behaviours at this level
• Can increase activity where 5HT is being produced - increase descending inhibition
• At the brainstem and periphery - downplay pain transmission

Side effects:

• Cardinal signs - respiratory depression, conscious depression, miosis
• Constipation, nausea and vomitting, anaphylaxis, tolerance and dependancy