Relistor (generic: methylnaltrexone) is a prescription medication used to treat constipation caused by prescription pain medicines called opioids (narcotics).  Relistor belongs to a group of drugs called peripherally acting mu-opioid receptor antagonists, which reverse the effects of opioids on the bowels, without affecting the pain relief from opioid treatment.

This medication comes in an injectable form and is given just under the skin once a day in the upper arm, abdomen, or thigh.

Common side effects of Relistor include stomach pain, gas, and nausea.