Best Heartworm Prevention for Dogs

How heartworm prevention works

Heartworms are transmitted by mosquito bites and, left unchecked, cause serious and potentially fatal heart and lung disease. Preventives don't repel mosquitoes — they work retroactively, killing the immature larvae a dog picked up in the previous month before they mature into adult worms. That's why consistent, on-time monthly dosing matters so much. In the US, heartworm preventives require a veterinary prescription, and dogs should be tested before starting and annually thereafter.

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Combination chews we like

Many owners prefer an all-in-one monthly chew. [Simparica Trio Chewable for Dogs](/products/simparica-trio-chewable-for-dogs) covers heartworm plus fleas, ticks, and certain intestinal worms in a single beef-flavored tablet. [Sentinel Spectrum Chews for Dogs](/products/sentinel-spectrum-chews-for-dogs) protects against heartworm and several worms and helps control fleas, while [Interceptor Plus Chewable for Dogs](/products/interceptor-plus-chewable-for-dogs) pairs heartworm prevention with broad intestinal-worm coverage. Your vet can help match the right combination to your dog and region.

The heartworm-only classic

If your dog already gets separate flea and tick protection, a heartworm-focused product may make sense. [Heartgard Plus Chewables](/products/heartgard-plus-chewables) is a long-established monthly chew covering heartworm plus roundworms and hookworms. Choosing between an all-in-one and a single-purpose product depends on what else your dog receives and your budget — a conversation worth having with your veterinarian, who prescribes these regardless of which you choose.

Testing, timing, and missed doses

Give preventives year-round in most climates, since a single lapse can leave a dog vulnerable. Never start a preventive in a dog that hasn't been tested — giving it to a heartworm-positive dog can cause dangerous reactions. If you miss a dose, give it as soon as you remember and call your vet; long gaps may warrant re-testing in six to seven months. Annual testing catches breakthrough infections early, when treatment is safer and more effective.