- Blood samples will be obtained for measurement of concentrations of methadone (methadone group) and morphine and its metabolites, morphine-3-glucuronide and morphine-6-glucuronide (morphine group), prior to administration of initial bolus dose (t = 0) and t = 5 min, 15 min, 30 min, 60 min, 2 hrs, 4 hrs and 6 hrs; thereafter, blood samples will be obtained for measurement every 6 hrs for up to 90 hrs according to availability of vascular access for phlebotomy.