Pharmacokinetics of butorphanol and evaluation of physiologic and behavioral effects after intravenous and intramuscular administration to neonatal foals.
Authors: Arguedas M G, Hines M T, Papich M G, Farnsworth K D, Sellon D C
Journal: Journal of veterinary internal medicine
Summary
# Editorial Summary: Butorphanol Pharmacokinetics in Neonatal Foals Butorphanol is routinely used in equine neonatal practice despite a lack of published safety and efficacy data specific to foals; Arguedas and colleagues addressed this gap by characterising the drug's pharmacokinetic profile and physiological effects in six healthy pony foals aged 3–12 days following intravenous and intramuscular administration at 0.05 mg/kg. Peak plasma concentration after IV dosing reached 33.2 ng/mL with a terminal half-life of 2.1 hours, whilst intramuscular injection yielded a lower peak of 20.1 ng/mL at approximately 6 minutes, with 66% bioavailability. Critically, neonatal foals demonstrated markedly different pharmacokinetics than adult horses—a distinction with important dosing implications—and whilst vital signs remained largely stable, behavioural observations revealed significant sedation alongside notably increased nursing behaviour, suggesting the drug's effects on feeding drive warrant consideration during clinical use. These findings provide evidence-based guidance for practitioners using butorphanol in sick or injured foals, though the small sample size and use of pony foals (rather than Thoroughbreds or warmbloods) means results should be extrapolated cautiously to clinical populations with different growth rates and metabolic profiles.
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Practical Takeaways
- •Butorphanol is safe for use in neonatal foals at 0.05 mg/kg with minimal adverse effects on heart rate, respiratory rate, or blood pressure
- •The increased feeding behavior and sedation with butorphanol may be beneficial for neonatal foal comfort and recovery, particularly post-operatively or in painful conditions
- •IM administration achieves therapeutic levels rapidly (5.9 minutes) making it practical for field use when IV access is difficult in foals
Key Findings
- •Terminal half-life of butorphanol in neonatal foals was 2.1 hours with C0 of 33.2 ng/mL after IV administration, differing from adult horses
- •IM butorphanol achieved Cmax of 20.1 ng/mL at Tmax of 5.9 minutes with 66.1% bioavailability
- •Butorphanol produced sedation and significantly increased nursing/feeding behavior with minimal effects on vital signs in neonatal foals