Summary Tramadol is a centrally acting analgesic drug extensively metabolized in animal species. Its clinical response is mainly due to the M1 metabolite, poorly produced in dogs. Grapefruit-juice can inhibit the metabolism of different drugs in animals and humans. The aim of the present study was to evaluate the pharmacokinetics of tramadol and its major metabolites after co-administration of tramadol and frozen-dried grapefruit-juice. A balanced cross-over study was used involving six male Beagle dogs. They were administered with tramadol alone (5 mg/kg) or with tramadol (5 mg/kg) plus frozen-dried grapefruit-juice (10 g). The plasma concentration vs time curves showed significant differences during the first 4 h following drug administration. Tmax was at 1.33 and 1.70 h following tramadol and tramadol plus frozen-dried grapefruit-juice treatment, respectively. Significant differences were also shown in Cmax (490 vs 270 ng/ml) and AUC (11,610 vs 5,890 h·h·ng/ml). Significant differences between the treatments were shown in all the M1 parameters reported. M2 and M5 did not show significant differences after both administrations. In conclusion, the frozen-dried grapefruit-juice was shown to affect the plasma concentrations of M1, despite them being well below those reported in humans
