Name a peripheral mu-opioid receptor antagonist used to treat opioid-induced constipation.

• A. Methylnaltrexone
• B. Naloxone
• C. Naltrexone
• D. Nalmefene

Answer: (A)

Opioid-induced constipation happens because mu-opioid receptors in the gut slow motility and secretions. To relieve this without affecting pain relief, you want a drug that blocks these receptors in the periphery but stays out of the brain. Methylnaltrexone fits this role because it is a peripherally restricted mu-opioid receptor antagonist; its quaternary ammonium structure keeps it largely from crossing the blood-brain barrier, so it reverses opioid effects in the gut without reversing analgesia. It is typically given subcutaneously to treat opioid-induced constipation in patients on long-term opioid therapy.

The other antagonists— naloxone, naltrexone, and nalmefene—cross the blood-brain barrier more readily and can reverse central analgesia or precipitate withdrawal if used systemically, making them unsuitable for treating constipation caused by opioids.