Opioids are μ-opioid receptor agonists (or partial/mixed agonists) used for moderate to severe pain, anesthesia, and palliative care.
They decrease nociceptive transmission at both spinal and supraspinal levels.
Receptor targets:
Cellular effect:
| Drug | Brand | Mechanism / Receptor Activity | Classification | Relative Potency (vs Morphine) | Key Clinical Pearls |
|---|---|---|---|---|---|
| Sufentanil | Sufenta | Full μ agonist | Full agonist | ~500x | Extremely potent; anesthesia |
| Remifentanil | Ultiva | Full μ agonist | Full agonist | 100–200x | Ultra-short acting (ester metabolism) |
| Fentanyl | Duragesic, Sublimaze | Full μ agonist | Full agonist | ~100x | No histamine release; renal safe |
| Alfentanil | Alfenta | Full μ agonist | Full agonist | 10–20x | Rapid IV onset |
| Hydromorphone | Dilaudid | Full μ agonist | Full agonist | 4–7x | Less histamine than morphine |
| Oxymorphone | Opana | Full μ agonist | Full agonist | ~3x | Potent oral agent |
| Oxycodone | Roxicodone, OxyContin | Full μ agonist | Full agonist | ~1.5x | Good oral bioavailability |
| Morphine | MS Contin | Full μ agonist | Full agonist | 1x (reference) | Histamine release; active metabolites |
| Hydrocodone | Hysingla ER | Full μ agonist | Full agonist | ~1x | Often combined with acetaminophen |
| Methadone | Dolophine | Full μ agonist + NMDA antagonist | Full agonist | Variable | Long half-life; QT prolongation |
| Tapentadol | Nucynta | Weak μ agonist + Norepinephrine reuptake inhibition | Dual mechanism | ~0.4x | Less serotonergic than tramadol |
| Tramadol | Ultram | Weak μ agonist + SNRI | Weak agonist | ~0.1x | Serotonin syndrome risk. Lowers seizure threshold |
| Codeine | — | Weak μ agonist (CYP2D6 prodrug) | Weak agonist | ~0.1x | Variable metabolism |
| Meperidine | Demerol | Full μ agonist | Full agonist | ~0.1x | Normeperidine → seizures |
| Drug | Mechanism | Classification | Key Feature |
|---|---|---|---|
| Buprenorphine | Partial μ agonist (high affinity) | Partial agonist | Ceiling effect; MOUD |
| Butorphanol | κ agonist / μ antagonist | Mixed agonist | Ceiling effect |
| Nalbuphine | κ agonist / μ antagonist | Mixed agonist | Dysphoria |
| Pentazocine | κ agonist / weak μ antagonist | Mixed agonist | Psychotomimetic effects |
Partial and mixed agonists: