Side Effects of Heartgard Plus

By Lea Holland
Heartgard Plus for dogs prevents heartworms, as well as treats and controls roundworms and hookworms. But owners should read all accompanying materials to be aware of any side effects the medicine may cause.

Three Types

There are two types of ivermectin, a chewable and a tablet. These pills eliminate the tissue stage of heartworm larvae. A chewable ivermectin/pyrantel (Heartgard Plus) also takes care of the heartworm larvae but also treats and controls roundworms and hookworms.


Dogs should be given one pill every 30 days. It is recommended for canines 6 weeks and older.

Check for Existing Heartworm

Dogs should be checked for heartworms before they are given Heartgard Plus. A mild hypersensitivity-type reaction of diarrhea may occur due to the dead or dying microfilariae inside the animal, according to Merial, who produces Heartgard.

Side Effects

The following side effects have been noted after using Heartgard: lethargy, vomiting, diarrhea, convulsion and hypersalivation.


When given an elevated dose level (more than 16 times the prescribed dose) of ivermectin, the active ingredient in Heartgard, collies may be more sensitive and show these signs: depression, drooling, excitability, stupor, coma and death.
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