Morphine is an analgesic drug used for the treatment of acute and chronic pain syndromes for cancer patients. Glucuronidation is a major pathway of the elimination of morphine in humans. Morphine is metabolized to 3-glucuronide (no analgesic effect) and 6-glucuronide (more potently analgesic than morphine) mainly by UGT2B7. In the present study, we investigated the inhibitory effects of a variety of drugs on the morphine glucuronosyltransferase activities in human liver microsomes. Twenty-one drugs including anticancer drugs, immunosuppressants, analgesics, anticonvulsants, antidepressants, antipsychotic drugs were selected in this study, because they are frequently co-administered with morphine. We found that 10 out of 21 drugs, tamoxifen, tacrolimus, diclofenac, carbamazepine, imipramine, clomipramine, amitriptyline, diazepam, lorazepam and oxazepam extensively inhibited the morphine 3- and 6-glucuronosyltransferase activities. Although some of the drugs are not substrates of UGT2B7, they would be potent inhibitors of UGT2B7. If patients receive morphine and these drugs simultaneously, the drug-drug interaction may change the levels of morphine and these glucuronides, resulting in altered analgesic efficacy and the risk of side effects. The results presented here will assist clinicians in choosing the proper drugs and/or dosages, and enable them to anticipate potential drug-drug interactions.