Bioavailability of Oral Ondansetron in Dogs

In the Literature

Garrick A, Zersen K, Gustafson D, Quimby J, Diaz A, Shropshire S. Bioavailability of oral ondansetron in dogs: a crossover study. J Vet Pharmacol Ther. 2026;49(1):17-21. doi:10.1111/jvp.70024

The Research …

Vomiting is a common presentation in dogs. Although identification and treatment of the underlying cause is essential, control of clinical signs is often necessary for patient comfort and clinical stabilization. Several antiemetics (eg, metoclopramide, maropitant, ondansetron) are available for dogs, with different mechanisms of action and varying efficacy depending on underlying pathophysiology. Although IV administration is preferred in hospitalized patients, oral antiemetics are often necessary for outpatient management or in cases in which financial constraints preclude hospitalization.

Ondansetron, a 5-HT₃ receptor antagonist, is a potent antiemetic with demonstrated antinausea properties.¹ Although the pharmacokinetics of ondansetron are well established, the oral formulation has not been adequately studied in dogs.

This randomized crossover study aimed to determine the bioavailability of oral ondansetron and provide insight into potential clinical utility. The study included 8 healthy dogs, all of which received both IV ondansetron (1 mg/kg) and PO ondansetron (1 mg/kg) with a 1-week washout period between treatments. Blood samples were collected at predetermined time points over 8 hours and analyzed for ondansetron concentration.

Oral administration resulted in markedly lower plasma drug concentrations compared with IV administration. Mean oral bioavailability was 5.2% ± 2.1%, which is consistent with prior reports that demonstrated poor oral bioavailability of ondansetron in dogs and other species.^2,3 Low bioavailability is likely attributable to extensive first-pass metabolism. Time to maximum plasma concentration was highly variable, and all dogs had undetectable plasma concentrations at various time points; 2 dogs had detectable concentrations only at the 8-hour measurement.

These findings raise concerns about whether oral ondansetron achieves consistent therapeutic concentrations, particularly for centrally mediated antiemetic effects; however, prior research has suggested that oral ondansetron may provide some clinical benefit, potentially through peripheral 5-HT₃ receptor antagonism within the GI tract.^2,4

This study adds to growing evidence that oral ondansetron can result in unpredictable systemic exposure in dogs. In cases in which reliable antiemetic or antinausea effects are needed, IV administration should be considered first.

… The Takeaways

Key pearls to put into practice:

• Oral ondansetron demonstrates poor bioavailability in healthy dogs (5.2% in this study), with systemic drug exposure ≈20 times lower than with IV administration, raising concern about efficacy, particularly for centrally mediated antiemetic effects.

• Oral absorption is highly variable and unpredictable. All dogs in this study had undetectable drug concentrations at various time points, and some dogs achieved detectable levels only once.

• IV administration should be prioritized when ondansetron is selected for antiemetic therapy. Although oral ondansetron may provide some benefit through peripheral GI 5-HT₃ receptor antagonism, the limited systemic distribution supports preferential IV use in most clinical scenarios.

• Future pharmacodynamic studies evaluating clinical nausea scores in sick dogs are needed to determine whether oral ondansetron can provide meaningful antinausea effects despite low plasma concentrations.