What is the function of methylnaltrexone in pain management?

Methylnaltrexone is primarily used to address opioid-induced constipation, which is a common side effect of opioid therapy for pain management. It functions as a peripherally acting mu-opioid receptor antagonist. By selectively blocking the action of opioids on the mu-opioid receptors in the gastrointestinal tract, methylnaltrexone effectively inhibits the slow motility caused by opioids. This means that while it does not block the analgesic effects of opioids in the central nervous system, it helps relieve the constipation symptoms that can arise from opioid use.

In the context of pain management, its role is significant because managing side effects like constipation improves the overall quality of life for patients receiving opioids for pain relief. Therefore, its primary function aligns with the correct answer, highlighting its impact on gastrointestinal motility rather than inhibiting the pain-relieving properties of opioids or requiring higher doses for effective pain control.