Which opioid is described as a partial mu agonist for mild to moderate pain?

• A. Buprenorphine
• B. Butorphanol
• C. Methadone
• D. Fentanyl

Answer: (A)

Partial mu agonism means the drug activates mu receptors but produces less than the full maximum effect, giving analgesia with a ceiling on both pain relief and respiratory depression. Buprenorphine fits this description: it binds very strongly to mu receptors and provides relief for mild-to-moderate pain, but its effects level off at higher doses, making it safer than full mu agonists. Its high receptor affinity also means it can block other opioids and can complicate switching from full agonists. In contrast, full mu agonists like methadone and fentanyl produce greater analgesia and carry a higher risk of respiratory depression, while butorphanol acts mainly on kappa receptors with mu antagonism, not as a partial mu agonist.