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Why is my dog vomiting, lethargic, and not eating after flea meds?

Updated On September 23rd, 2025

Pet's info: Dog | Cavalier King Charles Spaniel | Male | neutered | 10 years and 2 months old | 22 lbs

I have a 10 year old Cavalier King Charles Spaniel, male, who hasn't touched his food but is eating grass and throwing up. He is lethargic, has just gotten a clean bill of health from a vet (2/28) but was given two flea treatments, one to kill any fleas right away, and a chew to control them for 3 months.

2 Answers

Most Helpful Answer

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Answered By Linda G, MS, DVM

Veterinarian

Published on March 5th, 2018

Hello. It sounds like Charlie was given Capstar & Bravecto. However, if they were administered on the 28th, the Capstar was out of his system after 24 hours. Vomiting & loss of appetite are the 2 most common signs of the Bravecto, but it is still an extremely safe medication. So I am not sure we can blame it on the flea medications after this much time. If the vomiting has just started, withhold all food for at least 8-12 hours, & only allow small amounts of water, preferably as ice cubes. Restart food using 25% cooked chicken & 75% white rice when the vomiting has stopped. Give very small meals frequently rather than 1 or 2 big meals. If Charley seems comfortable, transition back to his regular diet after about 1 day or so. If the vomiting continues, or he shows any signs of fever, belly pain or diarrhea please see your veterinarian. Thanks for using Petco Pet Education Center, formerly Petcoach.

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Other Answers

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    Answered By Dr. Dayna, DVM, CVA

    Veterinarian, Certified Veterinary Acupuncturist

    Published on

    I am sorry to hear that Charlie isn’t feeling well. It is certainly possible that the flea medication has made him nauseated, but it is also possible that something else is going on. He is at an age where we do see problems such as kidney disease, Pancreatitis, and even cancer. I would highly encourage you to schedule a follow up appointment with your veterinarian. If nothing else they can give some anti nausea medication to make him feel better. You may consider reporting this to the flea medication company an a possible adverse event. Best of luck and Thank you for using Pet Coach.

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