Which of the following is true about the opioids methadone, hydromorphone, and fentanyl?

• A. They are mu agonists
• B. They are mu antagonists
• C. They are dissociatives
• D. They are benzodiazepines

Answer: (A)

These opioids share the same primary action: they activate the mu opioid receptor. The mu receptor is a G-protein–coupled receptor in the brain and spinal cord; when stimulated, it dampens pain pathways, producing analgesia. Activation of this receptor also drives common opioid effects like sedation, euphoria, and respiratory depression, as well as dependence. Methadone, hydromorphone, and fentanyl are all mu receptor agonists, which is why they produce these opioid-like outcomes. They’re not mu antagonists (which would block effects), not dissociatives (which act on NMDA receptors), and not benzodiazepines (which modulate GABA-A).