A study investigated the pharmacokinetic of acetaminophen dosed at 20 mg/kg in horses.  Eight healthy Thoroughbred geldings were given acetaminophen (20 mg/kg; 500 mg tablets) orally as a single dose followed by doses every 12 hours for 14 days. Only one dose level (20 mg/kg) was studied, sample size was small and only a single breed and sex was used, with no pre-treatment liver biopsies.

Serial blood samples were collected for determination of plasma acetaminophen concentrations using HPLC-UV. Serum biochemical analysis, gastroscopy, and liver biopsy were examined during the safety study. Following a single dose, mean maximum concentration was 16.61 μg/mL at 1.35 hours, and drug concentration was below the lower limit of detection in most horses by 24 hours. Elimination half-life was 2.78 h. No significant accumulation was noted following multiple doses. Average maximum concentration of acetaminophen following multiple oral dosing was 15.85 μg/mL and elimination half-life of 4 hours. Serum activities of sorbitol dehydrogenase were significantly decreased, and total bilirubin concentrations were significantly increased following the last dose. No statistically significant changes were noted in gastroscopy scores.

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Pharmacokinetics and safety of repeated oral dosing of acetaminophen in adult horses.