Methylnaltrexone (Relistor) is a subcutaneous, peripheral μ-opioid receptor antagonist used primarily to offset the detrimental effects of opioids on the gastrointestinal (GI) tract, including constipation and decreased GI motility. Due to its charged, quaternary structure, methylnaltrexone does not cross the blood-brain barrier and does not negate the analgesic benefits of opioids in the central nervous system.

Remember that although opioids confer pain relief primarily through central nervous system receptors, some of the analgesic benefits in inflammatory conditions are due to peripheral μ-opioid receptor activation. Methylnaltrexone will block these peripheral receptors and can therefore worsen the pain. Consequently, I’ll turn to this option after more conventional therapies: multimodal analgesia (limiting opioids), bowel regimens, ambulation, etc.

Drop me a comment below with your experiences and questions!

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  1. Been a nurse for 15 years, only given the drug once. The next day was one of the worst of my nursing career because the patient would not stop pooping, to the point where we had to put in a rectal tube to contain it because it was literally dripping off the bed. Granted, this was my one and only time ever giving it, but is there an in between the “ conventional modalities“ and this? Because this was awful.

    • Hard to really answer that as patients respond differently to therapies and the reason for the problem (constipation in this case) varies. For example, methylnaltrexone wouldn’t be helpful for isolated ileus unless there’s a concern of it being opioid-induced. In any case, I’ll start with things like early ambulation, enteral hydration, fiber, osmotic agents like MiraLAX, stimulants like Senna, lubricants like mineral oil, and enemas before going to methylnaltrexone.

    • My understanding is that opioid induced pruritus tends to be more of a central phenomenon and shouldn’t be readily addressed with a peripherally acting agent like methylnaltrexone. Here’s a study that looked into this: link here


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