A methyl derivative of noroxymorphone with selective, opioid-receptor antagonistic activity. Methylnaltrexone displaces opioids from peripheral opioid receptors in the gastrointestinal tract, the bladder, and the skin, resulting in decreases in opioid-related constipation, urinary retention, and pruritis, respectively. Methylnaltrexone does not cross the blood-brain barrier and does not affect the centrally-mediated analgesic effect of opioids.