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Should I Use Trifexis for Flea and Heartworm Prevention?

Written by: Dogster Team

Last Updated on January 11, 2024 by Dogster Team

woman hand applies Dog Flea Tick Drops to the skin of a cute red mixed breed dog

Should I Use Trifexis for Flea and Heartworm Prevention?

I have been getting lots of questions about Trifexis lately. It is a relatively new combined flea, heartworm, and intestinal worm treatment. Here are two messages representative of the bunch:

My vet is recommending I switch from Sentinel to the new Trifexis. My dog is 11 with severe allergies and has never had fleas. What are your thoughts on changing, just because it does more?


Jacksonville, FL

We have miniature Australian Shepherds who are not able to take Imervectin. They have been on Interceptor to control heartworm. Since the issue with manufacturing has caused massive shortages, we will run out soon. I noticed a TV ad today for Trifexis, which I thought had the same active heartworm preventative as Interceptor. Are you familiar with this new product?


Naples, FL

In general, I believe that if something isn’t broken it shouldn’t be fixed. If your dog is doing well with his current parasite protocol, there’s no reason to change things. However, I believe something needs fixing with Carol and Trudy’s dogs.


First, there is the supply issue with Sentinel and Interceptor. Sentinel is a heartworm preventative and also sort of a flea preventative. In reality, it doesn’t prevent fleas from infesting your pet, but it prevents them from breeding in the house. A related product, Interceptor, does not pretend to control fleas.

The main ingredient in both is milbemycin oxime, which is a very effective and safe agent for the prevention of heartworm and intestinal parasites. Heartworm can be fatal to dogs, and the best treatment for heartworm infestation is hard to come by. Some intestinal parasites can spread to people, especially children, and cause serious illness. Because it works well, I’m a fan of milbemycin oxime. But I’m no longer a fan of Sentinel and Interceptor.

Sentinel and Interceptor have been hard to get for many months now. They are manufactured by Novartis, and the company suffered great shame — and huge supply backlogs — when its main U.S. factory was closed because of quality control problems. Long story short: I wouldn’t give Sentinel or Interceptor to my pal Buster, and I don’t recommend them to anyone.

Carol, you mentioned something else that makes me think your dog would benefit from a change: severe allergies. Yes, you threw in the requisite statement that he’s never had fleas. But you live in prime flea country. And most (if not all) dogs with severe allergies are more severely allergic to fleas than to anything else. A dog needn’t “have” fleas to suffer major consequences from being intermittently bitten while he’s out on walks.

In other words, dogs with severe allergies should be on a really good flea preventative. Historically, that has meant Advantage or Frontline. The folks at Bayer and Merial, respectively, get upset when people say what I’m about to say, but I’m going to say it anyway: Advantage and Frontline just don’t seem to work like they used to. I have seen this in my patients. I also have seen it in Buster, who never has missed a dose of Frontline Plus, but who has developed major itching during flea season for the last two years (although he’s never had a visible flea infestation).


Carol, if your dog’s allergies aren’t completely controlled, I’d strongly recommend upgrading his flea prevention. Elanco makes a newish product, Comfortis, that is administered orally and works really well. It causes a small number of dogs to vomit (usually only the first time), but the owners of the overwhelming majority of dogs who tolerate the product love it. You can count me among the Comfortis-loving dog owners. Instead of a greasy Frontline mess each month, Buster gets a flavor tab (he thinks it’s a treat). And it seems to work better, as evidenced by his decreased itchiness.

The active ingredient in Comfortis is called spinosad. When sales took off, someone at Elanco had an idea that was bound to be profitable: Why not combine spinosad with milbemycin oxime? They did it, and Trifexis was born.

In my opinion, Trifexis offers the best flea preventative as well as the best heartworm and intestinal worm preventative, although Comfortis combined with Heartgard Plus will be equally effective.

Finally, Trudy, most experts state strongly that the dose of ivermectin in Heartgard Plus is safe for Collies, Australian Shepherds, and other so-called MDR1-susceptible breeds. However, I’m never opposed to erring on the side of caution. I recommend Trifexis for your dog as well.

Conflict-of-interest disclosure: I have none. I work as an emergency vet, which means that none of my income is derived from the sale of flea or heartworm preventatives.

Got a question for Dr. Barchas? Ask our vet in the comments below and you might be featured in an upcoming column.

(Note that if you have an emergency situation, please see your own vet immediately!)

Featured Image Credit: Dmitriev Mikhail, Shutterstock


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