Which of the following is an example of an opioid mixed agonist-antagonist?

Which of the following is an example of an opioid mixed agonist-antagonist?

• A. Nalbuphine
• B. Morphine
• C. Fentanyl
• D. Naloxone

Answer: (A)

The key idea is that an opioid mixed agonist-antagonist provides analgesia by stimulating one opioid receptor while blocking another, giving pain relief with a different side‑effect profile than pure mu agonists. Nalbuphine acts as an agonist at the kappa opioid receptor to produce analgesia, but it blocks the mu opioid receptor. That mu antagonism limits typical mu‑receptor–mediated effects like strong euphoria, high abuse potential, and especially respiratory depression, which often has a ceiling with these drugs. This makes nalbuphine a classic example of a mixed agonist-antagonist: effective pain relief comes from kappa receptor activation, while mu receptor blockade reduces unwanted mu-mediated effects. In contrast, morphine and fentanyl are pure mu agonists and produce potent analgesia but also dose‑dependent respiratory depression and other mu‑related side effects; naloxone is a pure mu receptor antagonist and does not provide analgesia, instead reversing mu‑receptor–mediated effects.

The key idea is that an opioid mixed agonist-antagonist provides analgesia by stimulating one opioid receptor while blocking another, giving pain relief with a different side‑effect profile than pure mu agonists. Nalbuphine acts as an agonist at the kappa opioid receptor to produce analgesia, but it blocks the mu opioid receptor. That mu antagonism limits typical mu‑receptor–mediated effects like strong euphoria, high abuse potential, and especially respiratory depression, which often has a ceiling with these drugs.

This makes nalbuphine a classic example of a mixed agonist-antagonist: effective pain relief comes from kappa receptor activation, while mu receptor blockade reduces unwanted mu-mediated effects. In contrast, morphine and fentanyl are pure mu agonists and produce potent analgesia but also dose‑dependent respiratory depression and other mu‑related side effects; naloxone is a pure mu receptor antagonist and does not provide analgesia, instead reversing mu‑receptor–mediated effects.