Methylnaltrexone, 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 Methylnaltrexone 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.
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