The effects of dose and diet on the pharmacodynamics of omeprazole in the horse.
Authors: Sykes B W, Underwood C, Greer R, McGowan C M, Mills P C
Journal: Equine veterinary journal
Summary
# Editorial Summary: Omeprazole Efficacy in Horses—Diet and Dose Matter More Than Previously Thought Conflicting evidence in the literature about omeprazole's effectiveness at suppressing equine gastric acidity prompted Sykes and colleagues to investigate how dose and diet influence the drug's ability to maintain elevated intragastric pH over a 24-hour period. Using six Thoroughbreds fitted with gastric tubes, the researchers measured stomach acid suppression continuously over six days under four conditions: two omeprazole doses (1 mg/kg and 4 mg/kg bodyweight daily) combined with two diets (high grain/low fibre or ad libitum hay). The 4 mg/kg dose proved superior to 1 mg/kg, and crucially, omeprazole performed significantly better in horses on high grain/low fibre diets, where it kept pH above 4 for substantially longer periods, whilst efficacy was "relatively poor" in hay-fed horses—a finding that contradicts the assumption of uniform drug efficacy across all management systems. The study also identified a cumulative dosing effect not previously documented in equine literature, suggesting that gastric pH control improves progressively over the five-day dosing period rather than reaching plateau immediately. For practitioners, this indicates that one-size-fits-all omeprazole protocols may be inadequate; dosing recommendations should account for each horse's diet and management system, with particular attention needed for animals consuming predominantly forage, who may require higher doses or alternative gastric acid management strategies.
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Practical Takeaways
- •Omeprazole dosing and efficacy cannot be standardized across all horses—diet type significantly affects response, with grain-based diets showing better suppression than hay-only diets
- •The 4 mg/kg dose provides superior acid suppression compared to 1 mg/kg, and cumulative dosing effects suggest improved control over multiple days of treatment
- •When prescribing omeprazole, consider the individual horse's management and feeding program; horses on forage-only diets may require higher doses or alternative/adjunctive therapies for adequate gastric protection
Key Findings
- •Omeprazole at 4 mg/kg bwt produced higher intragastric pH and greater time above pH 4 compared to 1 mg/kg dose
- •High grain/low fibre diet resulted in better acid suppression efficacy than ad libitum hay diet
- •A cumulative effect of daily omeprazole dosing was observed, not previously reported in horses
- •Overall omeprazole efficacy in raising ventral gastric pH was less than previously reported in literature