Adverse Reactions to Heartworm Medicine


Heartworm disease is a serious and preventable condition that can be very harmful (even fatal) for dogs. The condition is spread by a mosquito which carries the larvae of infecting heartworms. While all dogs are at risk, the severity may vary depending on your location. Your veterinarian will likely recommend placing your pup on a preventative program with a heartworm medication. Like any other medication, while there are substantial benefits, there is also the risk of adverse reaction. The following is a summary of the adverse reactions that each medication has reported to the FDA.

Heartgard and TriHeart Plus

Heartgard and TriHeartPlus both contain the active medication ivermectin. Adverse reactions include ataxia (coordination problems), staggering walk, diarrhea, vomiting, lethargy, depression, anorexia, mydriasis (extended time with dilated pupils), hypersalivation and convulsions. As noted in the prescribing information for both medications, there are specific breeds (like Australian Shepherds and Collies) which are prone to increased sensitivity.

Interceptor and Sentinel

Interceptor contains the active ingredient milbemycin oxime. Users reported adverse reactions like vomiting, anorexia, lethargy, depression, weakness, ataxia, hypersalivation and convulsions. Senitinel contains a combination of milbemycin oxime and lufenuron. Adverse reactions were reported like pruritus (severe itching), (hives), anorexia, diarrhea, vomiting, depression, lethargy, hypersalivation, convulsions, ataxia and weakness.


Revolution’s active ingredient, selamectin, is unique since it kills adult fleas, prevents flea or tick infestation, controls and treats ear mites and prevents heartworm disease. Clinical trials revealed occasional reactions like inflammation near the application site, site-specific alopecia, vomiting, diarrhea, lethargy, anorexia, hypersalivation, muscle tremors and tachypnea (rapid breathing). Post FDA Approval reports of additional reactions like urticaria, erythema (skin redness/inflammation), ataxia, pruritis, fever and rare reports of seizures.


Proheart 6 is an injectable heartworm medication (moxidectin) which prevents heartworm infection for up to six months. The medication was removed from the United States market in 2004 because of serious reports of adverse reactions and numerous deaths. In June of 2009, the FDA approved the re-emergence of ProHeart 6, provided the medication come with additional warnings and precautions and is only distributed by specially trained veterinarians. The medication should not be used for dogs that have any history of allergies, are sick or otherwise debilitated or have a history of weight loss. Adverse reactions include severe allergic reactions, changes in appetite, vomiting, diarrhea, pale gums, seizures, increased thirst/urination, bleeding, weakness, bruising and changes in activity levels. The medication has also been linked to blood and liver disorders.


Work with your vet to determine the best medication for your dog. Preventing heartworm infections is important for your dog’s long-term health but you also want to minimize your dog's risk of experiencing serious adverse reactions. Be sure to talk to your vet about any and all medications, vitamins or supplements that you give your dog to decrease the risk of adverse reactions. Before giving your dog any medication, research the active ingredients and potential side effects so that you know what to watch for.

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