Which statement describes methylnaltrexone's role in opioid-induced constipation?

• A. It is a peripheral mu-opioid receptor antagonist used to treat opioid-induced constipation.
• B. It is a central mu-opioid receptor antagonist.
• C. It is a non-opioid laxative.
• D. It inhibits GI secretions.

Answer: (C)

Opioid-induced constipation occurs mainly because opioids activate mu receptors in the gut, slowing peristalsis and altering secretion. To relieve this without negating pain relief, you want a drug that blocks those gut receptors but doesn’t enter the brain. Methylnaltrexone is a peripherally acting mu-opioid receptor antagonist; its structure prevents it from crossing into the central nervous system, so it reverses opioid effects in the GI tract while preserving central analgesia. By blocking peripheral mu receptors, it helps restore motility and stool throughput in the intestines. It is not a non-opioid laxative—the action hinges on opioid receptor antagonism restricted to the periphery—nor does it work by inhibiting GI secretions.