The active ingredient in a drug used to relieve certain side effects caused by treatment with opiods (pain killers similar to morphine), such as constipation (hard, dry stools), itching, and low urine flow. Methylnaltrexone binds to opioid receptors outside the brain and may block the side effects of opioid drugs without affecting their ability to relieve pain. Methylnaltrexone is a type of peripheral opioid receptor antagonist.