Methylnaltrexone for opioid-induced constipation in advanced illness.

Background: Constipation is a distressing side effect of opioid treatment. As a quaternary amine, methylnaltrexone, a mu-opioid-receptor antagonist, has restricted ability to cross the blood-brain barrier. We investigated the safety and efficacy of subcutaneous methylnaltrexone for treating opioid-induced constipation in patients with advanced illness.