Six healthy horses with normal eyes were included in a study to determine if acetaminophen given orally could provide therapeutic levels within the eye [Peraza, J.; Hector, R.C.; Lee, S.; et al. Ocular penetration of oral acetaminophen in horses. Equine Veterinary Journal Dec 2022; https://doi.org/10.1111/evj.13902].
The horses received 20 mg/kg of crushed acetaminophen tablets orally every 12 hours for six total doses. The medication was mixed with corn syrup in grain.
One hour after the final dose of acetaminophen, the researchers performed an anterior chamber paracentesis to collect 0.4 ml of aqueous humor. At the same time, blood was collected for serum sampling. The horses underwent the same collection procedures three months later. Serum and aqueous humor were also evaluated for eicosanoids (thromboxane B2, prostaglandins, F2-alpha, leukotriene B4, and 5-and 15-hydroxyeicosatetraneic acid).
Ocular Penetration of Acetaminophen
Acetaminophen penetrated the aqueous humor of normal eyes in this study. The concentration ratio of aqueous humor:serum was higher than what is seen with other NSAIDs—values of 45 compared to 4 for flunixin and 12 for firocoxib.
Characteristics of acetaminophen—low molecular weight and lipid solubility—allow it to pass through ciliary epithelial cells. Eicosanoids were not detected in the aqueous humor at either sample collection although they were detected in serum of some study horses. Absorption of acetaminophen occurs in the proximal small intestine. Therefore, it is affected by the rate of gastric emptying, which in turn is affected by meal size and composition.
Serum concentrations of acetaminophen were at or below what is considered the minimum therapeutic concentration in people; the therapeutic minimum concentration is not yet established in horses. However, other equine studies have demonstrated that oral acetaminophen doses at 20-30 mg every 12 hours provides satisfactory analgesia. By its nature, acetaminophen has only mild anti-inflammatory properties, so it might not provide this effect in the eye. The authors used it as adjunctive treatment for ocular pain with some success. More studies are warranted to determine if it can be an addition or alternative to traditional NSAIDs to manage ocular pain and inflammation.