42 patients admitted in ICU with intolerance to enteral feeding (GRV more than 250 ml) are recruited. All patients enter a primary acetaminophen absorption test study as baseline. Serum levels of acetaminophen will be measured by florescence polarization method at 15,30,45,60,90,120,180,240,480 minutes after enteral administration of 975 mg acetaminophen. Then the patients will be randomized to methylnaltrexone or erythromycin group.Another acetaminophen absorption test with the same schedule will be done after the last dose of each drug.The area under the curve for acetaminophen blood level will be used to compare the effect of two studied drugs on gastric emptying time.