BACKGROUND: Despite frequent clinical use, information about the pharmacokinetics (PK), clinical effects, and safety of butorphanol in foals is not available. OBJECTIVES: The purpose of this study was to determine the PK of butorphanol in neonatal foals after IV and IM administration; to determine whether administration of butorphanol results in physiologic or behavioral changes in neonatal foals; and to describe adverse effects associated with its use in neonatal foals. ANIMALS: Six healthy mixed breed pony foals between 3 and 12 days of age were used. METHODS: In a 3-way crossover design, foals received butorphanol (IV and IM, at 0.05 mg/kg) and IV saline (control group). Butorphanol concentrations were determined by high-performance liquid chromatography and analyzed using a noncompartmental PK model. Physiologic data were obtained at specified intervals after drug administration. Pedometers were used to evaluate locomotor activity. Behavioral data were obtained using a 2-hour real-time video recording. RESULTS: The terminal half-life of butorphanol was 2.1 hours and C0 was 33.2 +/- 12.1 ng/mL after IV injection. For IM injection, Cmax and Tmax were 20.1 +/- 3.5 ng/mL and 5.9 +/- 2.1 minutes, respectively. Bioavailability was 66.1 +/- 11.9%. There were minimal effects on vital signs. Foals that received butorphanol spent significantly more time nursing than control foals and appeared sedated. CONCLUSIONS AND CLINICAL IMPORTANCE: The disposition of butorphanol in neonatal foals differs from that in adult horses. The main behavioral effects after butorphanol administration to neonatal foals were sedation and increased feeding behavior.
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