Morphine is the first choice of treatment of severe post-operative pain, despite the occurrence of often discomforting (post-operative nausea or vomiting (PONV)) and sometimes dangerous (sedation, respiratory depression) side effects. Literature data indicate that morphine's active metabolite, morphine-6-glucuronide (M6G), is a powerful analgesic with a possibly more favourable side-effect profile. In this multi-centre randomised controlled clinical trial patients undergoing major abdominal surgery were randomised to M6G or morphine treatment. Treatment started 30-60 min prior to the end of surgery and was continued postoperatively, after patients were titrated to comfort, via patient-controlled analgesia (PCA) for 24-48 h. Pain intensity, nausea, vomiting and sedation scores were collected at regular intervals. In the study 268 patients were randomised to M6G and 249 to morphine. Withdrawal due to insufficient pain relief occurred predominantly just after surgery and was higher in the M6G group (16.8%) than in the morphine group (8.8%), suggesting a slower onset of analgesia for M6G compared to morphine. Subjects who continued on PCA remained equi-analgesic throughout the study. During the first 24h, nausea levels showed a 27% difference in favour of M6G which narrowly failed to reach statistical significance (P=0.052). Sub-analysis showed a significant reduction in nausea levels in females on M6G (30% difference, P=0.034). In all patients, similar reductions of 30-35% were observed in anti-emetic use, vomiting, PONV (a combined measure of nausea and vomiting) in favour of M6G, persisting for the first 24h postoperatively. Reductions in sedation were observed in the first 4h post-operative period for M6G patients. Copyright © 2010 European Federation of International Association for the Study of Pain Chapters. Published by Elsevier Ltd. All rights reserved.