Opioid receptor pharmacology and individual opioids
Opioid receptor signalling and where each effect comes from, full and partial agonists and antagonists, tolerance, and the kinetics, metabolites and interactions that set morphine, oxycodone, hydromorphone, fentanyl, codeine, tramadol, tapentadol, methadone and buprenorphine apart.
Tier 1Medical sciences concept
Written with AI assistance and fact-checked against the sources below. For exam study, not clinical decisions. How notes are made
At a glance
- Four opioid receptors: mu (MOP), delta (DOP), kappa (KOP) and nociceptin (NOP). Clinical opioids act mainly at mu.
- All are Gi/o-coupled GPCRs: less cAMP, potassium efflux (hyperpolarisation) and reduced presynaptic calcium entry (less transmitter release).
- Intrinsic efficacy separates full agonists (morphine, oxycodone, fentanyl, methadone), partial agonists (buprenorphine) and antagonists (naloxone, naltrexone).
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Sources
- Therapeutic Guidelines: Pain and AnalgesiaAU/NZTherapeutic Guidelines Limited, 2024
- Therapeutic Guidelines: Palliative CareAU/NZTherapeutic Guidelines Limited, 2024
- Australian Medicines HandbookAU/NZAustralian Medicines Handbook Pty Ltd, 2026
- PS01(PM) Appendix 2: Opioid dose equivalenceAU/NZFaculty of Pain Medicine, ANZCA, October 2025
- Clinical Pharmacogenetics Implementation Consortium guideline for CYP2D6, OPRM1 and COMT genotypes and select opioid therapyClin Pharmacol Ther (Crews et al., 2021;110:888-96), 2021
- Goodman & Gilman's The Pharmacological Basis of Therapeutics (14th ed.)McGraw Hill (Brunton and Knollmann, 2023), 2023
- Codeine pharmacokinetics: oral bioavailability relative to intramuscular dosingClinical Pharmacology and Therapeutics (Findlay JW et al., 1977), 1977
- Last updated
- 14 November 2026
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