Cumulative omeprazole administration is associated with increased hazard of exercise-induced pulmonary hemorrhage detection in Thoroughbred racehorses.

Cumulative omeprazole use is associated with increased EIPH risk, warranting careful reconsideration of prophylactic administration in racehorses.

This retrospective cohort study investigated associations between cumulative oral omeprazole administration and four clinical outcomes—exercise-induced pulmonary hemorrhage (EIPH), epistaxis, atrial fibrillation, and fracture—in 2,924 Thoroughbred racehorses in Hong Kong across the 2012–2013 and 2022–2023 racing seasons. Time-to-event analyses using Cox proportional hazards models incorporated cumulative omeprazole dose as a time-varying covariate, with adjustment for trainer, training intensity, racing year, and age at first trackwork. Results demonstrated that cumulative omeprazole administration during the preceding 30 days was significantly associated with an increased hazard of EIPH detection (HR 1.08 per 10-g increase; 95% CI, 1.01–1.15). A trend toward increased hazard of epistaxis was also observed (HR 1.20; 95% CI, 0.99–1.45), though this did not reach statistical significance. By contrast, omeprazole administration showed no meaningful association with fracture (HR 1.02; 95% CI, 0.89–1.17) or atrial fibrillation (HR 1.02; 95% CI, 0.86–1.20). The proposed mechanism linking omeprazole to EIPH may involve alterations in gastric acid suppression affecting systemic physiology, though causality cannot be confirmed from this observational design. These findings carry significant clinical implications, suggesting that prophylactic or prolonged omeprazole use in racehorses warrants careful reconsideration. Veterinarians should weigh the gastrointestinal benefits of omeprazole against potential pulmonary hemorrhage risks, particularly in horses with pre-existing EIPH susceptibility.

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