Methadone in healthy goats – Pharmacokinetics, behaviour and blood pressure

The pharmacokinetics and effects of the opioid methadone on behaviour, arterial blood pressure, heart rate and haematocrit were studied in goats. Two goats received methadone (0.2 mg/kg) intravenously and the terminal half-life was 88 and 91 min, the volume of distribution 8.4 and 6.1 L/kg, and clearance 86 and 123 mL/min/kg. In a crossover study eight goats received methadone (0.6 mg/kg) or 0.15 M NaCl subcutaneously (SC). After SC administration bioavailability was complete and the terminal half-life was 215 ± 84 min (mean ± SD), Tmax 31 ± 15 min and Cmax 45 ± 11 ng/mL. Blood pressure and haematocrit increased while heart rate did not change. The goats did not ruminate and they climbed, scratched, gnawed and showed tail-flicking after SC methadone in contrast to NaCl administration. The use of methadone in goats may be restricted due to the inhibition of rumination and the rather short half-life.