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.
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