The objective of this study is to test the intravenous and intramuscular administration of tramadol in alpacas (Vicugna pacos), to assess both its pharmacokinetic properties and its safety profile. The study design comprised a 2 groups, single dose, 2 treatments, 2 periods, randomised, open, balanced, cross-over design. Eight healthy male alpacas (Vicugna pacos), aged 5-9 years, and weighing 41-58 kg were selected. After both the administrations of tramadol (2.5 mg/kg), the concentration of tramadol and its main metabolites, M1, M2 and M5, were determined in plasma using an HPLC method. Moderate side-effects were shown after IV administration. The intramuscular bioavailability of the drug (81.5%) was within the generally accepted values for a positive bioequivalence decision of 80-125%. After the intramuscular injection the mean plasma drug concentration peak was reached after a Tmax of 0.16 h with a Cmax of 1.25 μg/mL. The minimal effective plasma concentration was reached after few seconds following intramuscular dosing and maintained for about 2-3 h in both administrations. The plasma concentrations of M1 and M5 were low and the amounts of the M2 produced were analogous in both routes of administration. In conclusion, tramadol was rapidly and almost completely absorbed after IM administration and its systemic availability was equivalent to the IV injection. The differences in the observed onset time and duration of action were very small and probably therapeutically irrelevant. Hence, IM injection of tramadol is a useful and better alternative to IV injection in alpacas
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