Summary Background The active metabolite of morphine, morphine-6-glucuronide (M6G), shows an analgesic effect that is equivalent to morphine. The bioavailability is low (11–13%) when given orally due to poor absorption from the GI-tract and further the absorption seems to be biphasic. The aims of this study were to determine the pharmacokinetics after intravenous administration (i.v.) of M6G in humans, to elucidate the absorption profile of M6G after oral administration, and to evaluate if the bioavailability could be increased by co-administration of the absorption enhancer Tween 20. Methods The study was a non-blinded randomised balanced three-way crossover study in eight healthy male subjects. The subjects received 25 mg i.v. M6G, 200 mg oral M6G, and 50 mg oral M6G + 500 mg of Tween 20. Blood samples were collected until 12 h after i.v. administration and until 24 h after oral administration. Paracetamol and sulfasalzine were co-administered with oral administrated M6G as markers for when the gut contents reached the duodenum and colon, respectively. Results The pharmacokinetic profile of i.v. M6G was in close agreement with previously reported data ( t 1/2 0.93 ± 0.2 h, CL 157 ± 8 ml/min, V d,ss 13.2 ± 3.5 l, and AUC 0− ∞ 5341 ± 289 nmol/l/h). After oral administration, neither bioavailability nor time to the first or second peak changed in the presence or absence of Tween 20 ( F: 9.3 ± 2.4 and 10.1 ± 4.8%; t 1: 2.5 [1.0–6.1] and 5.0 [2.0–8.2] h; and t 2: 18.0 [11.0–20.0] and 20.0 [11.0–24.0] h, respectively). Conclusion In this study, the pharmacokinetic profile of M6G was found to be in agreement with profiles found by others. Two different plasma concentration peaks were found after oral administration of M6G, the second of which occurred after the gut contents reached colon. Co-administration of Tween 20 did not enhance the bioavailability.