Trifexis Safety

Janet writes with an easy question, “Do you have any information on how well Trifexis works -side effects, effectiveness, etc.?”Janet, our acceptance of Trifexis Heartworm Preventive/flea control was made easy by the fact that it consists of two ingredients with long histories of safety and effectiveness.

Trifexis is heartworm prevention that includes flea control AND prevention for hookworms, whipworms and roundworms.
Trifexis is heartworm prevention that includes flea control AND prevention for hookworms, whipworms and roundworms.

Milbemycin oxime, the active ingredient in Interceptor Heartworm Preventive, has been available in United States markets for over two decades. Specific information on Interceptor may be viewed by visiting the
Novartis Animal Health Web site. Milbemycin oxime has the ability to provide heartworm prevention and to prevent three kinds of intestinal parasites: hookworms, roundworms and whipworms.

Milbemycin oxime has been shown, during both FDA approval processes, to provide the same protection in Trifexis that it does in Interceptor. FDA did not simply accept the prior studies and challenges, but required the Elanco company to prove that combining milbemycin oxime with spinosad, the active ingredient in Comfortis would not alter its ability to provide the same protection.

Comfortis’ package insert, on the Elanco Web site, shows the FDA’s required information.

The arrival of Comfortis was a happy day in our practice. For those who don’t want flea control medication on the outside of the pet where children might come in contact with it, Comfortis is the answer. For those who fear that either topicals or transdermals might get rubbed off onto furniture or carpet, Comfortis is the answer. For those who worry that their pets swim or are bathed too frequently for externally-applied products to work for a whole month, Comfortis is the answer.

We have been using Trifexis for just a few months as of this writing, but we have yet to see our first problem with it.

Based on Trifexis’ building blocks, we don’t expect any problems.

See you tomorrow, Dr. Randolph.


  1. used trifexis one time on minature schnauzers,one a four female died with in 24 hours, 10 year old male had seizures. has any one else reported this kind of a reaction?

    • I’m sorry, Rachel, but I must object to the term “reaction” in this case. Hundreds upon hundreds of things could cause a four year old female schnauzer to die. The fact that she died around the same time as Trifexis administration could be coincidence unless she had missed dose(s) of heartworm preventive and had contracted heartworms, known or unknown to you. Dogs with heartworms should never be administered heartworm preventive, except under the strict and direct supervision of a veterinarian. Space and time prevent us from listing the remainder of the hundreds of possible causes of death. Given that schnauzers are a breed overrepresented for idiopathic epilepsy (leading to seizures), it is not surprising to know of a 10-year old schnauzer experiencing them. The obvious question is, what physical and/or laboratory evidence is there to support Trifexis being the cause? And, perhaps most importantly, did you and/or your veterinarian report these events to Elanco as adverse events? If not, it’s not too late.

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