Methylnaltrexone: Peripherally Acting µ-Opioid Receptor Antagonist.

Opioid-induced constipation (OIC) is a common adverse effect associated with opioid therapy, with many patients never developing tolerance to this effect. There are many traditional laxatives available to help patients combat this symptom, yet OIC may not reliably respond to conventional treatment. Peripherally acting µ-opioid receptor antagonists (PAMORAs) have a place in the treatment of refractory OIC, when traditional laxatives have not resulted in effective laxation. There are a number of PAMORAs now available, and methylnaltrexone is the only PAMORA indicated for the treatment of OIC in adults with advanced illness, as well as for patients with chronic noncancer pain, including patients with chronic pain related to prior cancer treatment who do not require frequent opioid escalation. Advanced practitioners need to have an understanding of how and when to best use these medications for the different indications in patients with advanced illness or chronic noncancer-related pain.