Heartgard Plus Side Effects

Heartworms are microscopic parasites affecting canines, and are transmitted by mosquitoes. Heartgard Plus is a once-a-month beef-flavored chewable tablet containing the drugs ivermectin and pyrantal pamoate to prevent heartworms, and treat and control roundworms and hookworms. Heartgard Plus is available by prescription only through a licensed veterinarian. Speak to your veterinarian before using Heartgard Plus and in the event of the manifestation of side effects.

A dog is at the veterinarian. (Image: Jupiterimages/Stockbyte/Getty Images)

Common Reactions

Merial, the makers of Heartgard Plus, cite vomiting and diarrhea as the most commonly observed adverse reactions noted in clinical field trials of the drugs. As the most common side effects, these reactions were noted in only 1.1 percent of cases where the drug was administered, according to the product's safety and administration guideline pamphlet. Additional side effects of Merial included in the pamphlet are depression, lethargy, unwillingness to eat, loss of coordination, convulsions, hypersalivation and excessive dilation of the pupils.

Ivermectin Sensitivity

The "Clinical Textbook for Veterinary Technicians" notes that dogs of collie heritage are more sensitive to ivermectin, exhibiting dilated pupils, depression, lack of coordination, hyper-salivation, tremors, hyperactivity, paralysis, coma and death. The manufacturers of Heartgard Plus note this risk in their drug information pamphlet, but state that clinical testing of the drug in collies at even 10 times the recommended dosage failed to produce the effects listed. Merial states that Heartgard is safe for use in collies, but "Fundamentals of Pharmacology for Veterinary Technicians" recommends caution when using the product in collies or collie mixes. Observe any collie or dog of mixed heritage for at least eight hours as a precaution against adverse side effects, and seek emergency veterinary care immediately if adverse reactions present themselves.

Allergic Reactions

Allergic reactions can be seen in dogs with circulating microfilariae, the microscopic larvae of heartworms. The makers of Heartgard Plus note that Heartgard is not effective at clearing microfilariae from the dogs' systems and should not be used in dogs diagnosed as heartworm positive. Heartworm-positive dogs given Heartgard can experience what the manufacturer deems a "mild hypersensitivity-type" allergic reaction from the presence of dead and dying microfilariae. The most common indicator of this reaction is diarrhea, as noted by the manufacturer, though "The Veterinary Parasitology Reference Manual" notes that vomiting, depression, lethargy, hyper-salivation and tremors can also occur.