Methylnaltrexone binds to the same receptors as opioid analgesics such as morphine, but it acts as an antagonist, blocking the effects of those analgesics, specifically the constipating effects on the gastrointestinal trac. Furthermore, as methylnaltrexone cannot cross the blood-brain barrier, it does not reverse the pain-killing properties of opioid agonistsor or cause withdrawal symptoms, which are obviously of great benefit to the patient.