Medicine & USMLE

Full Opioid Agonists

Other Neuro Pharm
  1. Ramelteon
  2. Triptans
  3. Benzodiazepines - Function
  4. Zolpidem Zaleplon Eszopiclone
  5. Suvorexant
  6. Bromocriptine (Ergot Dopamine Agonists)
  7. Pramipexole, Ropinirole
  8. Amantadine
  9. Levodopa, Carbidopa
  10. Entacapone, Tolcapone
  11. Selegiline and Rasagiline
  12. Benztropine, Trihexyphenidyl
  13. Tetrabenazine
  14. Baclofen
  15. Memantine
  16. Riluzole
  17. Full Opioid Agonists
  18. Partial Opioid Agonists
  19. Dextromethorphan
  20. Tramadol
  21. Naloxone
  22. Naltrexone


Full opioid agonists are important medications used primarily for pain management or analgesia. Important drug names include methadone, heroin, codeine, morphine, and meperidine. These drugs work by reducing synaptic transmission of pain, by reducing presynaptic calcium influx and increasing postsynaptic potassium efflux -- therefore halting neurotransmitter release.

Apart from their analgesic effects, opioids can cause CNS and respiratory depression, constipation, and miosis. Many patients develop tolerance to opioid effects, and some even develop dependence.

Key Points

  • Full Opioid Agonists
    • Drug Names
      • Morphine, oxymorphone, hydromorphone
      • Heroin
      • Fentanyl
      • Codeine, oxycodone, hydrocodone
      • Methadone
      • Meperidine
    • Mechanism
      • Agonist at Mu Opioid Receptors
        • Also activator of delta and kappa opioid receptors
        • Resemble endogenous forms: beta-endorphins (mu agonists, derived from POMC which also makes ACTH), enkephalins (delta), and dynorphin (kappa)
        • Reduces synaptic transmission of pain neurons by:
          • Increasing potassium (K+) efflux
            • Opens postsynaptic K+ channels
            • Leads to hyperpolarization and termination of pain transmission
          • Reducing calcium (Ca2+) influx
            • Closes presynaptic Ca2+ channels
            • Prevents calcium-dependent vesicle fusion, which is required for neurotransmitter release at synapse
    • Indications
      • Analgesia (pain relief)
    • Adverse Effects
      • Constipation
        • Most common side effect
        • Tolerance does not develop
      • Miosis (pupil constriction)
        • Except meperidine, which causes mydriasis
        • Tolerance does not develop
      • Respiratory Depression
        • Common cause of death in opioid intoxication
      • Sphincter of Oddi spasm (biliary sphincter)
        • May lead to biliary colic due to increased pressure in common bile duct and gallbladder
      • CNS Depression
        • May lead to sedation or coma
      • Dependence
        • May cause euphoria and withdrawal when suddenly stopped