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When should my new puppy start heartworm medicine? Is testing needed?

Updated on September 23rd, 2025

Pet's info: Dog | Mixed Breed Small (up to 22lb) | Male | unneutered | 2 months and 3 days old | 3.5 lbs

We just got a 9 week old, 3.5 lb. Puppy. When should he start heartworm medicine, which one is the safest, and does he need tested first?

1 Answer

Most Helpful Answer

Jennifer Summerfield, DVM CPDT-KA

Answered By Jennifer Summerfield, DVM CPDT-KA

Veterinarian, Certified Dog Trainer

Published on June 22nd, 2019

Hi there. Your pup is old enough to start on heartworm prevention now. I would suggest using Heartgard or Sentinel - both of these are fine for a pup who is 9 weeks old and weighs 3.5 pounds, and both are very safe in general. Puppies under six months of age do not need to be tested first.
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