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My dog is sick after a missed Heartgard dose. Could it be heartworms?

Updated on September 23rd, 2025

Pet's info: Dog | Mixed Breed Large (61lb +) | Male | neutered | 7 years and 7 months old | 80 lbs

I haven’t given my oldest dog heart guard in a few months. I gave him a pill yesterday without being tested, and today he has thrown up and had diarrhea. He will not eat. Do you think he might have heart worms and is having some sort effect from taking the heart guard?

1 Answer

Most Helpful Answer

Linda G,  MS, DVM

Answered By Linda G, MS, DVM

Veterinarian

Published on March 18th, 2019

Hello. Heartguard is generally considered very safe, even in a dog with a current heart worm infection. If there was a reaction due to the presence of heartworms, it is almost always some kind of pulmonary event. So, it is possible that he has just developed a sensitivity or had a one off reaction to the Heartguard. Do think about having Buster tested before giving another pill, & do try to remember to give it every 30 days. All heartworm medications do a great job, but only if doses are not skipped, especially for dogs living in warmer climates. And if he does not seem better tomorrow, please make an appointment with your vet to have things checked out. Thanks for using Petco Pet Education Center, formerly Petcoach.
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