Ask a Vet: Does Trifexis Really Kill Dogs? Or Is It Just an Internet Rumor?

A TV news report says a common heartworm and flea preventative may be killing dogs -- but it has all the hallmarks of an unfounded rumor.


A while ago I was speaking with my editor at Dogster. I mentioned that I would like to write a piece on canine Internet rumors and how to detect them. These rumors are common. They also have a great deal in common with each other.

Most canine Internet rumors involve websites dedicated to stating that a product kills dogs. These sites usually contain heart-wrenching stories of tragic loss. The perpetrators of the rumors, in most cases, have not intentionally set out to start a false rumor. Rather, they honestly believe their stories. They have attributed or more frequently misattributed their dog’s death or illness to a product that their dog consumed or was exposed to (while ignoring many other possible causes for the illness or death).

The websites tend to be very emotional and emphatic. They exist for just about every product out there. I have seen such sites for Rimadyl, Advantage, Frontline, Purina, Science Diet, Vectra 3D, K9 Advantix, most vaccines, Nutro, and countless other products. In fact, if you want to experiment, hop on over to Google and search for “X” killed my dog. X can be anything. Something will probably come up (although I tried searching for “water killed my dog” and came up surprisingly short).

Now, emphatically, I am not making light of the potential for any product to cause illness or death in dogs. Dogs can have idiosyncratic reactions to any product. So yes, every product ever invented (as well as every natural product out there) has the potential to cause a fatal reaction in an especially susceptible individual. But for most products that are generally safe, reactions are far from the norm. Peanuts have the potential to cause fatal reactions in sensitive people, but that does not mean that peanuts kill people in general.

I understand why people are motivated to create sites warning of products that they perceive to be dangerous. If a person believes a product killed his dog, he will have a strong desire to spread the word. The millions of people whose dogs benefited from that same product don’t have such a strong incentive.

Another way to sniff out an unfounded rumor is to look at problems the offending product is purported to cause. Toxic agents act through specific mechanisms. Anticoagulant rodenticides cause uncontrollable bleeding. Non-steroidal anti-inflammatory agents can cause liver, kidney, and GI tract damage. Tainted melamine in dog food causes kidney failure. But many of the products about which Internet rumors spread are said to cause just about every problem under the sun. To quote toxicology expert Sharon Gwaltney-Brant on the Veterinary Information Network, “Oftentimes, especially for the drugs, you will see that the animals are having all kinds of problems; they are having cancer, cancer of the liver, cancer of the brain. They are having respiratory issues. They are having GI issues … If you have five animals all with five different types of disease, I don’t think I am going to be thinking too much about having a common toxic issue in those situations.”

So, to recap: Internet rumors generally start with true stories of sad loss, often incorrectly attributed to a product that was used. They tend to be highly emotional, they often aren’t backed up by facts found on autopsy, and the suspect product is often believed to lead to all manner of unrelated problems. There is one more thing: The people behind these rumors generally have unwavering belief in them.

Lo and behold, look what fell into my lap as I was contemplating this article. A television station in Florida ran a report on dog owners who believe that Trifexis (a combination flea and heartworm preventative) is killing their dogs.

The report has all of the hallmarks of an Internet rumor. It mentions a Facebook page titled “Trifexis Killed my Dog.” It is highly emotional — one of the people interviewed for the story was quoted as saying, “It’s like a piece of your heart is being torn out.” Trifexis is purported to cause many different problems in dogs. One dog suffered neurological symptoms including vertigo, lethargy, and seizures. Others suffered inflammation of the heart. And peoples’ belief in the menace of Trifexis is unwavering, even in light of evidence to the contrary: An independent autopsy on one of the dogs mentioned in the story “stated a bacterial infection likely caused the dog’s heart failure. It ruled his symptoms were not typical of drug toxicity.” The owner was not convinced.

Again, emphatically, I am not making light of the true suffering of the people in the story, nor the tragic loss of the dogs mentioned in the story. Nor am I saying that it is impossible that Trifexis was involved in any of the deaths.

Trifexis contains two active ingredients. One, milbemycin, has been used for many years as a heartworm preventative sold under the brand names Interceptor and Sentinel. It can, in fact, cause neurological symptoms in especially sensitive dogs. All heartworm preventatives have this potential, but a dog would need to be extraordinarily sensitive to experience any side effects at the doses used in Trifexis.

The second ingredient in Trifexis is the flea preventative spinosad. This product also is available as Comfortis. It is widely used, and it causes a not insignificant proportion of dogs to vomit. For the large majority of dogs who do not vomit (including my pal Buster), it appears to be by far the most effective flea preventative out there.

Could Trifexis cause heart failure? Anything is possible, I suppose. But I’ve never heard of a confirmed case. In fact, Trifexis is much more likely to prevent heart failure than to cause it, since it prevents heartworm disease.

Is Trifexis rampantly killing dogs? The answer to that is clear. It is not. And that is why I don’t like Internet rumors. Just as the now completely discredited purported link between childhood vaccines and autism is estimated to have caused the death of over 1,000 children, I worry that the Trifexis rumor could cause dogs to die of heartworm disease and to suffer from miserable skin problems as a result of flea allergies.

Everything in life carries risks and benefits. Walking your dog comes with the risk of him slipping his harness and getting lost. Playing fetch carries the risk of choking on the ball. Every medication and treatment carries the risk of side effects. Before starting your dog on Trifexis or any medication, explore those risks. And also explore the benefits. But do it responsibly, and avoid websites with agendas.

Note to readers: I do not own stock in Elanco. I do not even receive a discount on Buster’s Comfortis.

Read more by Dr. Eric Barchas:

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!)

10 thoughts on “Ask a Vet: Does Trifexis Really Kill Dogs? Or Is It Just an Internet Rumor?”

  1. NOW that we’re in 2020, the FDA and good vets have warnings out about these so called preventives! Who would have figured!

  2. My pointer mix began showing signs of extreme itchiness after I first gave him trifexis. Never had any other symptoms. No lethargy, vomiting, tremors, mood change or anything. He just became PERMANENTLY itchy all the time. The pill got rid of the fleas but left my poor boy with the constant need to scratch. It breaks my whole heart.

  3. I’m a vet. My 2 year old poodle reacted to trifexis after the monthly dose including significant tremors ataxia and weakness. It resolved in 12 hours. I’m now using Bravecto without problems.

  4. Our Catahoula has been on Trifexis for 5 years at the 40-60lb dose. She did not seem to have any side effects that we could tell other than she threw up after a couple of doses for sure, in hindsight she may have thrown up more times but not immediately after, so we did not put two and two together. After our recent visit to our vet she weighed 62 pounds and was prescribed the 60-120 dose. I thought that was extreme for only being 2 pounds into that range and thought about asking about it, but I didn’t they are the professionals not me. We still had one of the old pills left so she didn’t get the new dose for a month after the Vet visit.
    When we gave her the pill with a full meal, she ate it with no problem as she always does. She is not on any other meds and is healthy as can be the only catalyst was the Trifexis. We let her sleep with us that night as we do from time to time, so I know she didn’t puke until sometime the next day.
    In the morning she was very lethargic and didn’t really want to go outside which is very unusual, but with some coaxing she did anyway to please us. the whole rest of the day all she did was sleep never wanted to play like she always does, just basically knocked out all day. I did find a pile of vomit outside on the patio, we have a doggie door she can go out there whenever she wants, so I’m not sure what time she vomited.
    Wasn’t too concerned until it was dinner time and she didn’t even stir when I was getting it ready, which never happens she normally gets all excited dancing around and barking for me to hurry up, instead she just stayed in her bed looking lazily at me. I put her bowl down and called for her to come over and she wouldn’t move, put the bowl right next to her by her bed she wanted nothing to do with it. That’s when I got really concerned, she lives for dinner time!
    I then Googled Trifexis side effects and started reading the horror stories and got really concerned. Called Elanco and they took the lot number of the meds and some other info and said to monitor her if she gets worse call an emergency Vet as it was 9pm by now. She was so sleepy and hardly reacting to me or anything I did to her she just barely cracked open her eyes for a half second before closing them again back dead asleep, very comatose.
    Called emergency Vet and they told me this is a reaction to the Trifexis and try and feed her some chicken breast and monitor her gums as long as they stay pink she should be o.k. if she does not improve by morning take her to the Vet first thing. I finally got her to eat half of a boiled chicken breast shredded up in her dry food and she also began to eat the dry food as well, this made me feel better for the night, after eating she immediately went back to sleep.
    I stayed next to her all night to make sure she was breathing. In the morning she was tired, this was after 36 straight hours of sleep since she took the pill, but after her walk she was almost back to normal and wanted to play again and she did her normal sleep/play routine throughout the day. Third day now and she seems totally back to normal, thankfully! Very unsettling thing to go through and I’m not sure I will give her Trifexis again especially after reading stories that are way worse than mine.

    1. My dogs dosage went up as well because she was closer to 10 pounds. the last two times her symptoms have worsened. Today she vomited but now she’s shaking and lethargic. The shaking stopped after I warmed her up. She’s refusing an ice cub. I’m going to have to call tomorrow.
      I have the same issue when I take the doctor prescribed ibuprofen, so she must be sensitive. She’s sensitive to some of her vaccines as well.

  5. My healthy 18 month old bull Terrier had a severe reaction to Trifexis beginning a few hours after her monthly dose. She had tremors, lethargy and arching of the back and neck just as many dogs on YouTube suspected of the same reaction. Called our local emergency vet and they said to watch her for a few hours and if she didn’t improve we should bring her in. They said it sounded like a reaction to Trifexis and that they had seen a lot of these. They said it usually cleared up within a few hours. Our dog was markedly improved after 7-8 hours and is fine 3 days later. No doubt in my mind that this was a direct result of the Trifexis. Our dog is very robust and healthy. Had she been very old or young I don’t doubt that the episode could have been life threatening.

  6. As of late 2017 this article is now out of date, mostly incorrect and appears to have been commercially biased.

    In response to anecdotal evidence and ongoing legal action, spinosad-based products now come with a warning from the manufacturer not to be used on animals susceptible to seizures or in combination with ivermectin.

    Research at Washington State University supported that animals with the MDR1 genetic mutation are susceptible to potentially fatal side effects from these medications, either immediately or through tissue buildup. Tests for the gene are now available across most of the world.

    The FDA approval report states these medications cause long-term buildup in kidney, liver, central nervous system and brain tissue of animals and exposed humans (from spot-on treatment) that have unknown long-term effects.

    Long term human impact studies on spinosad have concluded it has deleterious effects on fertility and early to teen childhood development, with clear links to developmental problems, Autism and ADHD in children 0 to 15yrs. These effects were previously noted in extreme animal lab testing by the manufacturer.

    Fipronil, used in other products, is under Government review in Australia and other places as unsafe. Scientific evidence suggests its’ environmental impact includes breaking down into 5 more dangerous chemical substances that accumulate in the food we eat and stay in the soil for years.

    You don’t have to believe my conclusion, the best and worst thing is references official, scientific and anecdotal are just a quick search away to gather the information and find your own.

  7. My dog (Australian Shepherd, 5 years old) became paralyzed 3 days after I gave her a dose of Trifexis. She was unable to walk or lift her head. I brought her to an emergency vet who contacted Animal Poison Control. They quickly dismissed my claim and stated that it must be due to some “undetermined unrelated preexisting condition”. The vet discharged my dog without treatment. She remained paralyzed for close to 3 weeks. After which time, she was able to walk fine. She never experienced any kind of paralysis prior to this event, and in the two years since then she has never experienced any kind of paralysis after. She is a happy, healthy dog who has yearly full blood panels and all routine vaccinations and checkups.

    The random, sudden paralysis was 100% certainly due to the Trifexis. She had not taken any other medication or visited any strange places at the time. However, experiences like mine from other pet owners are quickly swept under the rug or ignored. I wish I’d listened to these so-called “internet rumors” before I gave my furry child something that hurt her so badly.

    1. I wish I had seen all the info before I gave my beautiful Nory her first dose yesterday. She was fine for at least 6 hours after then I went to work & came home 6 hours later to a dog that couldnt seem to get up (still can’t) & is nothing like her usual healthy self. I carried her to van last night & took her to the emergency clinic where they said best guess is a reaction or soreness to the rabies vaccine (never fazed her before) shes a bit worse this morning but is at least taking water that I carry to her

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