Pharmacokinetics and thermal anti-nociceptive effects of oral morphine in horses.
Authors: Knych Heather K, Steinmetz Stacy J, Traynham Megan L, McKemie Daniel S, Kass Philip H
Journal: Frontiers in veterinary science
Summary
Morphine's efficacy as an equine analgesic is complicated by dose-dependent neuroexcitation and gastrointestinal disturbances when administered intravenously, yet oral dosing achieves comparable concentrations of the active metabolite morphine-6-glucuronide without triggering these adverse effects—though its anti-nociceptive properties via this route remained uncharacterised until now. Knych and colleagues investigated the thermal anti-nociceptive response to single and repeated oral morphine doses in horses, measuring both plasma pharmacokinetics and withdrawal responses to standardised thermal stimulation. The oral route produced M6G concentrations equivalent to therapeutic IV dosing, with anti-nociceptive effects demonstrable on thermal testing and sustained across multiple doses without apparent tolerance development or unwanted behavioural signs. For equine practitioners, these findings suggest oral morphine represents a practical alternative to IV administration for pain management, particularly valuable in field settings or where prolonged analgesia is required, whilst avoiding the neuroexcitation and colic-related complications that limit intravenous use. Further work establishing optimal dosing intervals and efficacy against other pain modalities would strengthen clinical application of this route in equine practice.
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Practical Takeaways
- •Oral morphine offers an effective analgesia option for horses without the neuroexcitation and GI side effects seen with IV dosing, making it potentially safer for field use and routine pain management
- •The anti-nociceptive effects of oral morphine are measurable and reproducible, supporting its use in clinical pain management protocols
- •Multiple dose regimens appear feasible, suggesting oral morphine could be incorporated into multimodal analgesia strategies for horses with acute or chronic pain
Key Findings
- •Oral morphine administration in horses produces comparable morphine-6-glucuronide (M6G) concentrations to IV administration without neuroexcitation or gastrointestinal adverse effects
- •Single and multiple oral doses of morphine demonstrated thermal anti-nociceptive effects in horses
- •Oral morphine provides a viable alternative to IV administration for equine analgesia with improved tolerability