3. Ondansetron (Zofran) Originally developed for humans undergoing chemotherapy, Ondansetron is sometimes used off-label for dogs, particularly in cases of severe nausea. It works by blocking serotonin receptors in the brain that trigger vomiting. While it can be highly effective, it is not as commonly prescribed as other anti-nausea medications, so it’s crucial to consult with your veterinarian regarding its use and dosing.