Pharmacokinetics and clinical efficacy of acetaminophen (paracetamol) in adult horses with mechanically induced lameness.
Authors: Mercer Melissa A, McKenzie Harold C, Byron Christopher R, Pleasant Robert S, Bogers Sophie H, Council-Troche Roberto M, Werre Stephen R, Burns Travis, Davis Jennifer L
Journal: Equine veterinary journal
Summary
# Editorial Summary: Paracetamol Efficacy in Equine Lameness Whilst paracetamol has gained traction as a musculoskeletal analgesic in equine practice—often used alongside traditional NSAIDs—its pharmacokinetic profile and clinical efficacy in horses remained poorly characterised. This 2023 study investigated plasma concentrations and anti-lameness effects of two paracetamol doses (20 and 30 mg/kg orally) against phenylbutazone and placebo in nine mares fitted with reversible mechanical lameness-inducing horseshoes, using a randomised Latin square design with blinded lameness assessment. The higher paracetamol dose (30 mg/kg) achieved peak plasma concentrations of 30.02 μg/ml within 25 minutes and demonstrated significantly improved lameness scores at 2 and 4 hours post-administration compared to placebo; notably, it also produced lower heart rates than phenylbutazone at multiple timepoints (2–8 hours), suggesting superior analgesic effect within the acute phase. However, no statistically significant lameness differences emerged between the two paracetamol doses and phenylbutazone when directly compared, and the study's reliance on subjective lameness scoring with a small sample size limits generalisability to clinical populations. For practitioners considering paracetamol monotherapy, these findings suggest rapid onset of action at 30 mg/kg but highlight the critical need for further research on repeated-dosing safety and pharmacokinetics before clinical protocols can be confidently established.
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Practical Takeaways
- •Acetaminophen at 30 mg/kg may offer faster pain relief in acutely lame horses compared to traditional NSAIDs like phenylbutazone, with effects visible within 2-4 hours
- •This small study suggests acetaminophen could be a useful alternative or adjunct therapy for acute lameness, but safety and efficacy of repeated dosing requires further investigation before routine clinical adoption
- •Heart rate response differed between treatments, suggesting acetaminophen 30 mg/kg may have additional physiological effects beyond analgesia that warrant further study
Key Findings
- •Acetaminophen 30 mg/kg achieved peak plasma concentration of 30.02 μg/ml within 0.43 hours, compared to 20.01 μg/ml at 0.66 hours for the 20 mg/kg dose
- •Acetaminophen 30 mg/kg produced significantly lower heart rate compared to acetaminophen 20 mg/kg, phenylbutazone, and placebo at multiple time points (1-8 hours post-treatment)
- •Lameness scores improved significantly with acetaminophen 30 mg/kg versus placebo at 2 and 4 hours post-treatment; phenylbutazone showed significant improvement only at 8 hours
- •No significant differences in lameness improvement were detected between acetaminophen 20 mg/kg, acetaminophen 30 mg/kg, and phenylbutazone when directly compared