Plasma concentrations of morphine, morphine-6-glucuronide and morphine-3-glucuronide and their relationship with analgesia and side effects in patients with cancer-related pain.

Morphine, the recommended drug for the management of moderate to severe cancer pain, is metabolized predominantly to the glucuronides morphine-6-glucuronide (M6G) and morphine-3-glucuronide (M3G). The quantitative clinical importance of these metabolites following the administration of oral morphine is unclear. This study investigates the relationship between plasma concentrations of morphine (M), M6G, M3G and clinical effects in patients receiving sustained release oral morphine for cancer-related pain.