Dangers With Heartworm Medications for Dogs


Treatment for heartworms in dogs have come a long way in recent years, but there are still some dangers. There are two basic types of medications. The preventatives kill the heartworms in their larval stage before they can do any harm. The treatment for ridding the dog of adult heartworms uses a different medication. Both are fairly safe, but can have side effects that depend on the breed of the dog.

About Heartworms

  • Dogs get heartworms when bitten by infected mosquitoes. When these infected mosquitoes bite dogs, the microscopically small larvae enter the bloodstream. The larvae lodge in the heart and lungs, where they can slowly grow to over a foot long and then breed. Over time, as the worms grow and multiply, dogs' heart muscles get weakened and their lungs slowly get obstructed. Without treatment, infected dogs will eventually die, usually from a sudden heart attack.


  • The most common heartworm preventative medication is ivermectin (HeartGard), a broad spectrum insecticide that is particularly effective for preventing heartworm infection when given orally once a month. Milbemycin oxime (Interceptor) is another monthly preventative. A previous preventative called ProHeart was taken off the market for causing seizures and deaths in many dogs.

Ivermectin Risks

  • Many herding breeds and some sighthounds carry a genetic mutation that can cause a fatal neurotoxic reaction to ivermectin (see the link in Resources for commonly affected breeds). It is safer to give these dogs Interceptor. There is also a DNA blood test available if you own one of the affected breeds.

Heartworm Treatment

  • The only medicine for a heartworm-positive dog is immiticide, an arsenic-based compound administered by injection. While arsenic-based medications may seem extreme, they are the only compounds known to kill adult heartworms. The danger from these medications stem primarily from the effects of the heartworms dying and breaking down in the bloodstream, where they can lodge in the heart in a clump and cause a heart attack. However, it is extremely likely that a heartworm-positive dog will die from the heartworms themselves, making the treatment an acceptable risk.


  • If you search the Internet for heartworm treatments, you may find advice to simply give large doses of ivermectin to kill adult heartworms, or to use black walnuts or other herbal remedies. Many of these sites are also selling the remedy they are touting. Understand that, glowing testimonies aside, none of these treatments have been proven to work, and some can even be dangerous. It is wisest to rely on an experienced veterinarian for advice and treatment.

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