Use of methylnaltrexone and related compounds

A method for preventing or treating opioid induced side effects including dysphoria, pruritus and urinary retention and non-opioid induced changes in gastrointestinal motility. The method comprises administering methylnaltrexone or another quaternary derivative of noroxymorphone to a patient prior to the administration of an opioid or after the onset of side effects induced by the administration of an opioid, wherein the methylnaltrexone or quaternary derivative is administered by the route selected from the group consisting of intravenous, intramuscular, transmucosal, transdermal, and oral administration, preferably administered orally in an enterically coated form.