Opioids
Opioids: Overview, Mechanism, and Clinical Use
Opioids are a class of drugs that either derive from natural substances found in the opium poppy (Papaver somniferum) or are synthetic/semi-synthetic compounds that mimic these natural alkaloids. They are primarily used for their potent analgesic (pain-relieving) properties but also have effects on mood, respiration, gastrointestinal function, and the central nervous system.
Mechanism of Action
- Opioids exert their effects by binding to specific opioid receptors located mainly in the central and peripheral nervous systems and the gastrointestinal tract.
- There are three classical opioid receptor types: mu (μ), kappa (κ), and delta (δ) receptors, all of which are G-protein coupled receptors. A fourth receptor, the nociceptin/orphanin FQ peptide receptor (NOP), is related but considered non-opioid.
- Binding to these receptors modulates neuronal activity by:
- Presynaptically inhibiting calcium channels, reducing neurotransmitter release (e.g., substance P, glutamate) involved in pain transmission.
- Postsynaptically opening potassium channels, hyperpolarizing neurons and decreasing excitability, which inhibits nociceptive signal propagation.
- The μ-opioid receptor mediates most analgesic effects, as well as euphoria, respiratory depression, and physical dependence. κ and δ receptors contribute to analgesia and other effects but with different profiles.
- Some opioids (e.g., methadone) also antagonize NMDA receptors, which may enhance efficacy in neuropathic pain.
- Certain opioids (e.g., tramadol, oxycodone, fentanyl) can influence serotonin pathways, which can increase the risk of serotonin syndrome when combined with other serotonergic drugs.
Pharmacological Effects
- Analgesia: Relief of moderate to severe pain via central and peripheral mechanisms.
- Euphoria and sedation: Effects on mood and consciousness.
- Respiratory depression: A dose-limiting and potentially fatal side effect.
- Gastrointestinal effects: Reduced motility leading to constipation.
- Physical dependence and tolerance: With prolonged use, leading to withdrawal symptoms upon cessation.
- Other effects: Cough suppression, miosis (pupil constriction), and hormonal changes.
Clinical Uses
- Management of acute and chronic pain, including cancer pain, postoperative pain, and neuropathic pain (some opioids).
- Anesthesia adjuncts.
- Treatment of opioid use disorder (e.g., methadone, buprenorphine).
- Cough suppression (e.g., codeine, dextromethorphan).
- Diarrhea treatment (e.g., loperamide).
Examples of Opioids
- Natural alkaloids: Morphine, codeine.
- Semi-synthetic: Oxycodone, hydrocodone, hydromorphone, buprenorphine.
- Synthetic: Fentanyl, methadone, tramadol, tapentadol.
Safety and Risks
- Risk of addiction, abuse, and overdose.
- Common side effects: nausea, vomiting, constipation, sedation.
- Serious risks include respiratory depression and potential for serotonin syndrome with certain opioids.
- Naloxone is an opioid antagonist used to reverse overdose.
Summary Table
| Aspect | Details |
|---|---|
| Drug Class | Natural, semi-synthetic, synthetic opioids acting on μ, κ, δ opioid receptors |
| Mechanism | G-protein coupled receptor agonists; inhibit neurotransmitter release and hyperpolarize neurons |
| Primary Effects | Analgesia, euphoria, sedation, respiratory depression, constipation |
| Clinical Uses | Pain management, anesthesia, opioid use disorder treatment, cough suppression, diarrhea control |
| Examples | Morphine, codeine, oxycodone, fentanyl, methadone, tramadol, buprenorphine |
| Risks | Dependence, tolerance, addiction, overdose, respiratory depression, serotonin syndrome risk |
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References
- Deer T, et al. Opioid Analgesics. StatPearls. 2023.
https://www.ncbi.nlm.nih.gov/books/NBK459161/ - Chahl LA. Opioids – mechanisms of action. Aust Prescr. 1996;19(3):63-65.
https://doi.org/10.18773/austprescr.1996.063 - Opioid – Wikipedia. 2025.
https://en.wikipedia.org/wiki/Opioid - Trescot AM, et al. Basic opioid pharmacology: an update. Pain Physician. 2012;15(3 Suppl):ES1-ES14.
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4590096/ - Johns Hopkins Medicine. Opioids.
https://www.hopkinsmedicine.org/health/treatment-tests-and-therapies/opioids - Mayo Clinic. What are opioids and why are they dangerous?
https://www.mayoclinic.org/diseases-conditions/prescription-drug-abuse/expert-answers/what-are-opioids/faq-20381270
Opioids remain essential drugs for managing moderate to severe pain but require careful use due to their potential for dependence, tolerance, and serious adverse effects. Understanding their mechanisms helps optimize therapeutic benefits while minimizing risks.
