Pharmacokinetics, clinical efficacy and safety of acetaminophen (paracetamol) in adult horses with naturally occurring chronic lameness.
Authors: Mercer Melissa A, Davis Jennifer L, McKenzie Harold C, Byron Christopher R, Kelleher Maureen E, Trager Lauren, Cecere Thomas E, Wilson Katie E, Council-Troche R M, Werre Stephen R
Journal: Equine veterinary journal
Summary
# Acetaminophen for chronic equine lameness: efficacy and safety profile established Researchers administered paracetamol at 30 mg/kg orally every 12 hours for 21 days to 12 horses with naturally occurring chronic lameness, measuring plasma drug concentrations via LC-MS/MS whilst evaluating lameness through both body-mounted inertial sensor (BMIS) technology and subjective scoring, supported by clinicopathological analysis, hepatic biopsy and gastroscopy to assess safety. Peak plasma concentrations were achieved rapidly (within 24–40 minutes) on both day 7 (20.83 ± 10.25 μg/mL) and day 21 (17.33 ± 6.91 μg/mL), with subjective lameness scores showing significant improvement at 2 and 4 hours post-treatment; hindlimb lameness specifically demonstrated measurable improvements in peak dynamic force asymmetry (PDmax) during the initial 8-hour window post-dosing. Importantly, 21 days of treatment produced no clinically significant hepatotoxicity, gastric ulceration or clinicopathological changes, establishing a favourable safety profile at this dosage and duration. The transient nature of the analgesic effect and lack of sustained improvement suggest paracetamol functions as a short-term adjunctive therapy rather than monotherapy for chronic equine lameness, potentially useful for acute flare-ups or as part of multimodal pain management strategies, though practitioners should recognise the limited evidence base in this naturally lame population and consider the rapid onset-offset kinetics when integrating treatment into clinical protocols.
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Practical Takeaways
- •Acetaminophen at 30 mg/kg twice daily is safe for 21 days with no hepatic or gastric toxicity concerns in chronic lameness cases
- •Short-term subjective improvement occurs within 2-4 hours post-dose, but this transient effect suggests acetaminophen alone is insufficient for managing chronic lameness
- •Consider acetaminophen as an adjunctive rather than standalone therapy, combining with other pain management strategies for better clinical outcomes
Key Findings
- •Acetaminophen 30 mg/kg PO every 12 h achieved Cmax of 20.83 ± 10.25 μg/mL on day 7 with Tmax of 0.40 ± 0.22 h
- •Subjective lameness scores significantly improved at 2 and 4 h post-treatment, with significant PDmax improvement in hindlimb lameness at 1, 2, and 8 h post-treatment
- •No clinically significant changes detected in clinicopathological analysis, hepatic biopsy, or gastric ulceration scores following 21 days of treatment
- •Acetaminophen produced only transient improvement and may not be effective as monotherapy for chronic lameness