Methylnaltrexone (Relistor) is the methylated, quaternaryform of naltrexone . The permanently chargednitrogen prevents the drug from crossing the blood-brainbarrier. Thus, it only acts as an antagonist at peripheralopioid receptors. Relistor was approved in April 2008 totreat opioid-induced constipation in patients receiving palliativecare. It is administered as a SC injection once everyother day.
