When to Start Heartworm Meds in Dogs

Mosquitoes can infect your dog with heartworms.
Stockbyte/Stockbyte/Getty Images

If your dog spends any time outdoors, mosquito bites may put him at risk for heartworm infection. The key to preventing heartworm disease is knowing when to begin treatment and maintaining a constant defense against these parasites.


Typically, you should begin heartworm treatment in puppies around 6 to 8 weeks of age, although some heartworm preventatives are suitable for even younger dogs. Once a dog reaches 6 months of age, however, you should not begin a preventative cycle without first having your dog tested for infection. Heartworm preventatives require your dog to be heartworm-free in order to avoid a potential anaphylactic reaction.


The frequency of heartworm medication depends on where you live. In northern climates, you may need to give your dog heartworm preventives only in the spring and summer, when mosquitoes are active in your region. In the South, and particularly near the Mississippi River, mosquitoes can be a year-round nuisance, so vets generally recommend giving preventives year round. Consult with your vet to work out a prevention plan tailored to protect your pet.