Evidence that morphine is metabolized to hydromorphone but not to oxymorphone

A minor pathway for the biotransformation of morphine to hydromorphone has been identified in humans. Recently, an unsubstantiated claim that morphine is metabolized to hydromorphone and then to oxymorphone was published. The goal of this study was to determine if credible evidence that oxymorphone is a metabolite of either morphine or hydromorphone exists. Urine specimens from pain patients who were treated exclusively with high daily doses of morphine (N = 34) or hydromorphone (N = 26) were analyzed by liquid chromatography-tandem mass spectrometry for oxymorphone, hydromorphone, and morphine (LOD = 25 ng/mL). Specimens were also tested for a variety of other medications. Criteria for inclusion of patients' specimens were as follows: 1. patients were undergoing exclusive dosing with either morphine or hydromorphone; 2. non-prescribed medications were not detected; and 3. urine concentrations of morphine were > 100,000 ng/mL for the high-dose morphine group and > 1,000 ng/mL of hydromorphone for the high-dose hydromorphone group. Consistent with earlier reports, hydromorphone was detected in patients treated with high-dose morphine. The ratio of hydromorphone to morphine ranged from 0.2 to 2.2%. Oxymorphone was not detected in any specimen from high-dose morphine or high-dose hydromorphone patients. The authors conclude, based on these data, that oxymorphone is not a metabolite of morphine or hydromorphone.