Does Trifexis Kill Dogs?



Updated: January 28, 2020

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A lot of people have emailed us asking this question recently, and the safety of Trifexis is certainly a very hot topic on social media right now (the topic even has dueling Facebook communities!). But is the viral nature of this topic a good thing? Or is it just fueling an unnecessary fire — creating distress and uncertainty for dog owners, and health and safety risks for dogs (and the people they live with)? Let’s dig a bit deeper.

The most popular news report on this matter currently is from Jim Strickland of WSB-TV 2 [link to story no longer active] in Atlanta, Georgia. Thankfully, this report is a (fairly) well-balanced one.

In this most recent report, Mr. Strickland starts by stressing that “there is no direct evidence linking Trifexis to any dog’s death,” a point which he thankfully repeats throughout the story. He even ends the report with the (very important) point that, regardless of which preventative is chosen, all dogs should be on some form of heartworm prevention. The overall tone of the story, however, does seem to play on emotions and border on alarmist - and all without scientific evidence to support the claims. And the station’s graphics department certainly takes some sensationalized liberties throughout the report — note the “965 Deaths” graphic at 1:25, where the word “complaints” is sadly omitted, or the selective use of capitalization in the graphic at 1:43. Overall, a fairly well-balanced report. However, it doesn’t really paint a full or clear picture. Let’s take a look at what’s known and what’s being claimed, as this is certainly an important topic where some clarity and perspective is in order.

Here are the key “take aways” from this specific report, once the sensationalism is removed:
  • There is currently no direct evidence linking Trifexis to any dog’s death — just pet owner suspicion.
  • Elanco and the FDA are currently looking into any possible connection between Trifexis and the reported deaths.
  • Elanco has previously been responsive to updating their labeling on Trifexis to reflect other potential side effects that were reported and confirmed following Trifexis launch (ataxia and seizures).
  • There are currently 968 complaints of dog deaths suspected to have resulted from Trifexis administration.
  • Over 70 million doses of Trifexis have been administered to dogs since the product came on the market.
  • Fergus, the main dog in the story, was 10 years old when he was given his first and only dose of Trifexis and had preexisting neurologic disease (“he had been living comfortably with an underlying nervous system condition common to Scottish Terriers“).

Unfortunately there are some very important things missing from this news report, too. These pieces of information are important in trying to determine a pattern and likely causation. Here’s what’s missing…
  • A more complete medical picture. Fergus’ full medical history… especially his recent history, including the date and results of the last veterinary exam and screening blood/urine tests that were performed on him prior to the administration of Trifexis. Whether or not he had any physical exam signs, historical suggestions, and/or biochemical markers of existing liver (or other) disease prior to being given Trifexis are truly important points of consideration.
  • Similarly, a more complete medical picture of the bulldog who developed seizures and was subsequently euthanized could help to clarify the connection, if any.
  • Necropsy results from either dog, if performed, would also be crucial to any investigation. Without them it would be very difficult to say conclusively if there is a connection or causation here.
  • Is there a potentially-unifying pattern amongst the dog deaths that have been reported? Were they all affected at roughly the same date and/or in the same geographic location — perhaps indicating a contaminant or otherwise bad lot? Are such deaths being reported in the other countries where Trifexis is used? Are the dogs all dying/being euthanized for the same reasons — the report states that Fergus suffered liver failure, while the bulldog suffered seizures. Certainly liver failure can, and often does, result in seizures — but was that the case here, or were the seizures a result of primary neurologic disease or another non-neurologic disorder? If there is no underlying pattern the likelihood of there being an underlying common cause — Trifexis or otherwise — is less.
Reports and news stories such as this are important as they help to raise awareness to problems. What’s also important to keep in mind though is that while a quick spread of such awareness can be a good thing, it can also prove to be distressing, and even dangerous, if the suspicion such stories is based on never pans out. The potential negative effects of such awareness, should it prove unfounded, can be lessened though by reading carefully between the lines, maintaining perspective, and asking the right questions before acting on the awareness. This is what I suggest at this point in the case, while the appropriate investigations are underway. Don’t forget, aside from you, your veterinarian and their team know your pets the best. Talk to them as you try to sort through the facts, fiction, and “yet to be decideds” in this case.

To that end, some very important things to keep in mind while this all plays out are the facts that we know about heartworms and heartworm preventatives:
  • Heartworm (Dirofilaira immitis) infestations debilitate and kill dogs - and cats, too.
  • Cats and dogs get heartworm disease when they are bitten by a mosquito carrying an early lifecycle stage of the heartworm.
  • Heartworm disease is present and has been diagnosed in all 50 states of the US.
  • Heartworm disease is preventable through mosquito control and the use of regular preventatives (of which, Trifexis is one - and a very effective one at that — but there are several others, too).
  • Though we colloquially refer to Trifexis and other such medications within this class as “heartworm preventatives,” “flea and heartworm medications,” or something similar, most of them actually do far more. Many of these types of medications actually repel, kill, and prevent not just heartworms and/or fleas, but also common intestinal parasites (e.g. roundworms, hookworms, and others), and potentially even ticks, mites, biting flies, and a host of other nuisance and potentially disease-transmitting pests.
  • An established case of heartworms in dogs can be treated, but it’s often both time and financially intensive to do so. Such treatment is not without its own risk, including the potential risk of death during treatment.
  • Safe and effective treatment is not currently available for cats with established heartworm infection.
  • The FDA thoroughly evaluates the results and claims of manufacturer’s studies on efficacy and safety of all oral medications we veterinarians prescribe and dispense for animals before these medications ever reach the market. They (both the FDA and the drug manufacturer) continue to monitor and investigate suspected concerns with all medications following licensing - as is happening in this case.

What does this all mean? What messages and lessons do I hope you will take from these reports and this post?
  • Heartworm disease is a nasty, debilitating, and potentially-fatal disease that everybody should take appropriate steps to prevent.
  • All cats and dogs (even indoor-only pets) should be on a regular, consistent, and complete parasite prevention plan that is chosen based on a conversation with, and a medical history review and examination conducted by, their trusted veterinarian.
  • All pets, even those on regular preventatives, should be tested for heartworm infection every year (it’s relatively inexpensive and can further help prevent pet illness and/or death).
  • There is currently a suspicion, though no conclusive proof, that there may be some significant safety problems with one of the most popular heartworm preventatives — Trifexis.
  • It appears as though these concerns are being taken seriously and are currently under investigation by the appropriate people and agencies.
  • Pet owners should report any suspected adverse events — with any medications — to their veterinarian, the drug manufacturer, and to the FDA (Information and resources for Adverse Event Reporting).
  • All medications (and even all supplements) can have unintended side-effects. There are many factors that influence these, and some become more evident as a medication is used more.
  • Based on their past responsiveness, it would seem that Elanco would “do the right thing” and update their labeling on and/or pull Trifexis if any of these deaths are confirmed to have been caused by Trifexis.
  • While the investigation is ongoing, you shouldn’t panic and you shouldn’t jump to conclusions - especially if your dog has been safely on Trifexis for some time now. However, If you do decide to change preventatives — which is your right and there are lots of other effective medications out there for you and your veterinarian to choose from — you should be sure to do so only with the counseling and input of your veterinarian. This is not a rash decision to be made based solely on a magazine ad, news report, Facebook post, or a conversation with an employee at your local pet store. Nor should it be based solely on what you read in a blog post — even this one. This is a medical decision, and therefore it should be made only after careful consideration and in conjunction with your pet’s veterinarian.

Links for more information and resources on this important topic:

PS - For what it’s worth, my own dog, Wendy, has been on Trifexis for over 2 years now. Prior to that she was on a combination of Comfortis and Sentinel (the same two active ingredients in Trifexis, spinosad and milbemycin oxime, respectively — though made by different companies) for well over a year. I just gave Wendy her monthly dose of Trifexis this past Friday.