What is "methylnaltrexone"

n. A peripherally-acting μ-opioid antagonist that acts to reverse some of the side effects of opioid drugs such as constipation without affecting analgesia or precipitating withdrawals.

Methylnaltrexone (MNTX, brand name Relistor), used in form of methylnaltrexone bromide ( INN, USAN, BAN), is one of the newer agents of peripherally-acting μ-opioid antagonists that act to reverse some of the side effects of opioid drugs such as constipation without affecting analgesia or precipitating withdrawals. Because MNTX is a quaternary ammonium cation, it cannot cross the blood–brain barrier, and so has antagonist effects throughout the body, counteracting effects such as itching and constipation, but without affecting opioid effects in the brain such as analgesia. However, since a significant fraction (up to 60%) of opioid analgesia can be mediated by opioid receptors on peripheral sensory neurons, particularly in inflammatory conditions such as arthritis, traumatic or surgical pain, MNTX may increase pain under such circumstances.