Pharmacokinetics of fentanyl following intravenous and transdermal administration in horses.

Summary

# Fentanyl Pharmacokinetics in Horses: IV and Transdermal Routes Fentanyl's unpredictable central nervous system effects in horses have limited its clinical adoption, yet transdermal patches were being used empirically for moderate to severe pain management without robust pharmacokinetic data to guide dosing or predict efficacy. Maxwell and colleagues compared fentanyl disposition following intravenous bolus and 72-hour transdermal patch application in horses, measuring plasma concentrations and deriving key pharmacokinetic parameters to establish bioavailability and clearance rates. The transdermal route proved substantially slower in achieving peak concentrations than IV administration, with steady-state levels reached over several days rather than minutes—a critical distinction for practitioners managing acute versus chronic pain scenarios. These findings provided the first evidence base for transdermal fentanyl use in horses, enabling clinicians to predict therapeutic windows and understand why CNS excitation risk differs markedly between IV and patch administration. For pain management protocols, this work justifies transdermal patches as a viable alternative where sustained, moderate analgesia is required, whilst highlighting the need for careful patient selection and monitoring during the initial absorption phase before therapeutic levels stabilise.

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Key Findings

• •Fentanyl transdermal therapeutic system (TTS) has been used empirically in equine medicine despite reported CNS excitation effects
• •Pharmacokinetic characterization of transdermal fentanyl administration in horses was needed to understand drug disposition and optimize clinical use for analgesia

Conditions Studied