Evaluation of Oral Acetaminophen on Tear Production and Intraocular Pressure in Healthy Dogs

OBJECTIVE: To assess the effects of a therapeutic dose of oral acetaminophen on Schirmer tear test I (STT-1) and intraocular pressure (IOP) in healthy dogs. ANIMALS STUDIED: Fourteen healthy adult beagle dogs. PROCEDURES: All animals underwent a physical and ophthalmic examination, hematology, and plasma biochemistry prior to treatment. Oral acetaminophen at 30 mg/kg every 12 h for 5 days was administered. STT-1 and IOP were measured in both eyes before drug administration. Ocular adverse effects were assessed using a semiquantitative preclinical ocular toxicology scoring system. All measurements were performed by the same investigator under controlled environmental conditions. At the end time point, follow-up physical and ophthalmic examinations and blood workup were conducted. RESULTS: STT-1 remained stable throughout the study, with no significant differences between time points (p = 0.665) or overall trend over time (p = 0.356). IOP showed no consistent temporal trend (p = 0.602), although significant differences were observed between specific time points (p = 0.003). IOP at 24 h was higher than at 12 h (p = 0.031) and 60 h (p = 0.009), and at 0 h compared to 60 h (p = 0.043); however, these differences were transient and clinically irrelevant (1-2 mmHg). Mild conjunctival hyperaemia and fluorescein staining uptake were observed in 3/28 eyes, although these were mild and resolved spontaneously without treatment. No systemic adverse effects were detected. CONCLUSIONS: Oral therapeutic doses of acetaminophen for 5 days did not affect STT-1 or IOP in healthy dogs. Further studies should evaluate its effects in dogs with systemic diseases, pre-existing ocular conditions, or as long-term or multi-drug regimen treatment.