Why I Discourage Simparica Trio & Other Isoxazolines — And What to Do Instead

When a client comes to me with a dog who suddenly started having seizures, or who has been “off” since starting their flea and tick prevention, one of the first questions I ask is: What flea and tick product is this dog on? More often than not these days, the answer involves a drug in the isoxazoline class — Simparica Trio, NexGard, Bravecto, Credelio or others.

These products are aggressively marketed. Pharmaceutical reps push them. They are actually pesticides. They’re convenient. And yes, they do kill fleas and ticks. But as an integrative veterinarian who takes a whole-body, long-view approach to canine health, I have serious reservations about recommending them — and I want to be transparent with you about why.

Let me walk you through the concerns, from your dog’s neurological health, to the risks to the broader ecosystem, to the important but often glossed-over fact that these products don’t actually prevent tick bites at all.

The FDA’s Seizure Warning: What It Means for Your Dog

In 2018, the U.S. Food and Drug Administration issued a formal safety alert that should have generated much more public discussion than it did. The FDA required manufacturers of all isoxazoline-class products to update their labeling to include explicit warnings about neurological side effects — including muscle tremors, loss of coordination (ataxia), and seizures.

⚠ Official FDA Warning

The FDA has stated that isoxazoline products “have been associated with neurologic adverse reactions, including muscle tremors, ataxia, and seizures in some dogs and cats.” Critically, the agency noted that these neurological events “may occur in animals without a prior history” of seizure disorders. Another study has highlighted the incidence of adverse reactions in 2020.

That last part is what concerns me most as a clinician. We’re not just talking about dogs who were already seizure-prone. We’re talking about dogs who had no history of neurological issues, who had one dose of a flea prevention chew, and who then experienced their first seizure. Post-marketing adverse event data gathered by the FDA and the European Medicines Agency (EMA) revealed that the number of serious reactions — including seizures and deaths — was substantially higher than what had been reported during pre-approval clinical trials. A peer-reviewed analysis comparing FDA and EMA data found a seven to ten times higher rate of seizures and deaths reported in Europe compared to U.S. reports for the same products.

The mechanism isn’t mysterious. Isoxazolines work by blocking GABA-gated chloride channels in insect nerve cells, causing paralysis and death in the flea or tick. The premise is that these channels are more selective for invertebrate nervous systems than for mammalian ones. But GABA is a critical inhibitory neurotransmitter in dogs’ brains too — and the emergence of neurological signs across the entire drug class tells us the selectivity is not absolute. When those channels are partially blocked in your dog’s brain, seizures can result.

The drug stays in your dog’s system for weeks to months — that’s the whole point of the long-acting formulations. (And with the new injectable product, Bravecto Quantum, a whole year!) That means if a neurological reaction occurs, there is no way to quickly clear the drug from the body. There is no antidote.

The Pesticide Chart: Know What’s In Your Dog’s Chew

There are now many isoxazoline products on the market, including combination formulas that bundle multiple active ingredients into one chew. Here’s a breakdown:

The Problem with Combination Products

Let’s talk specifically about Simparica Trio and similar “all-in-one” combination products, because this is where I think the conversation becomes especially important.

Simparica Trio contains three active ingredients: sarolaner (the isoxazoline for fleas and ticks), moxidectin (a macrocyclic lactone for heartworm prevention and some intestinal parasites), and pyrantel (a dewormer). The marketing is straightforward — one chew, one dose, everything covered. Convenient, yes. But here’s the problem: one size does not fit all.

Consider Your Region & Season

In the northeastern United States, mosquitoes — the insect that transmits heartworm — are inactive for many months of the year. How many mosquitoes have you seen when there is snow on the ground? A dog in upstate New York or Vermont who is on Simparica Trio through January and February is receiving monthly heartworm prevention that is, in those months, biologically unnecessary. Yet they are still receiving a full systemic dose of sarolaner and its associated neurological risks every single month, year-round, to cover for a threat that isn’t present. Ticks and fleas are also far less common in winter months. In summary, these products are a waste of your money and an unnecessary risk for your dog in these situations.

The principle here is foundational to integrative veterinary medicine: never give a drug that isn’t needed. Every pharmaceutical has costs — metabolic burden, potential adverse effects, financial cost to you, and, as we’ll discuss next, environmental cost. When a combination product bundles ingredients together for the sake of convenience, it removes your ability to make targeted, seasonal, risk-appropriate decisions for your individual dog.

A better approach for many dogs, particularly those in lower-heartworm-risk areas or in regions with true winters, is to use targeted, single-ingredient products only when the specific risk exists — and to use heartworm prevention only during the actual mosquito season. Talk with your holistic veterinarian about what makes sense for your dog and your region.

Isoxazolines Don’t Actually Prevent Tick Bites

This is one of the most important — and least discussed — limitations of this entire drug class, and it should weigh heavily on your decision-making.

Isoxazolines are not repellents. They cannot deter a tick from landing on your dog, crawling through the fur, or attaching. The only way the drug reaches a tick is through the dog’s bloodstream, which means the tick must bite your dog first, begin a blood meal, and ingest enough of the circulating drug before it is killed. The manufacturers have often stated something to this effect, if not totally claiming the tick would not stay attached long enough to transmit disease.

“For fleas and/or ticks to be killed, they have to bite the dog first and suck enough blood before the ingested active ingredient kills them. Whether killing is fast enough to prevent disease transmission is not yet known for all parasites and diseases.”

Real-world experience has taught us otherwise. When a dog has a tested negative for years, then switched to an isoxazoline product, and subsequently tests positive for Lyme, anaplasmosis, or other tick borne disease, what other conclusion can be drawn?

For Lyme disease, the Borrelia burgdorferi bacterium is said to require 24 to 48 hours of tick attachment before meaningful transmission occurs. Some isoxazolines kill ticks quickly enough to beat this window in controlled laboratory studies. But for other diseases, the picture is more complicated. Ehrlichia canis transmission can begin within just 3 to 6 hours of attachment by brown dog ticks — a timeline that even rapid-acting isoxazolines may not consistently beat. The controlled studies showing disease prevention have not been replicated for all tick species, all pathogens, and most importantly in all real-world conditions. In my exam room, dogs taking isxozolines commonly test positive for Lyme and anaplasmosis.

Daily tick checks remain essential even on these products. Which raises a reasonable question: if you’re doing thorough daily tick checks anyway, are there safer ways to manage tick risk?

The Hidden Environmental Cost: Your Dog’s Waste is Toxic to Insects

If the risks to your dog’s nervous system weren’t enough to give you pause, consider what these drugs do after they leave your dog’s body.

Isoxazolines are primarily eliminated through the fecal route — meaning the active drug compounds pass through your dog’s gut and are excreted in their waste in biologically active form. A landmark 2026 study published in Environmental Toxicology and Chemistry by researchers at VetAgro Sup in France examined fecal samples from dogs and cats treated with isoxazoline products over a three-month period.

Researchers detected active isoxazoline compounds — particularly fluralaner (Bravecto) and lotilaner (Credelio) — in pet feces after the end of the recommended treatment period. The estimated median half-life of fluralaner in dog feces was nearly 23 days. Their environmental risk assessment, concluded that dung-feeding insects could be highly exposed to isoxazoline compounds, with “potentially disastrous consequences for environmental lifecycles.”

Who are these dung-feeding insects? Dung beetles, dung flies, and certain butterfly and moth species that use animal waste as a food source or breeding substrate. These insects are not pests. They are critical components of healthy ecosystems — essential for breaking down waste, recycling nutrients back into soil, improving soil structure, and controlling other pest populations. Without them, ecological processes we depend on begin to break down.

The European Medicines Agency had previously highlighted the risk of isoxazolines contaminating ecosystems. UK environmental studies have found companion animal pesticides — including related compounds — in a significant proportion of river and surface water samples, at concentrations toxic to aquatic insects like dragonflies and mayflies. Some sources worry about the impact on fish and even the birds that may prey upon them. The scale of isoxazoline use among pet owners globally makes the fecal excretion pathway a genuine ecological concern — one that has received almost no attention in mainstream veterinary practice.

When you give your dog Simparica Trio or Bravecto, you are not just making a health decision for your dog. Every time your dog eliminates in a yard, a park, or a field, you are potentially exposing the insects that live in and depend on that soil to a potent neurotoxic insecticide. For those of us who care about pollinators, soil health, and ecosystem integrity, this is a meaningful part of the calculus.

Tick Awareness: A Smarter, Safer Approach

I want to be clear: I take tick-borne disease seriously. Lyme disease, anaplasmosis, ehrlichiosis, and Rocky Mountain spotted fever are real threats to dogs in many parts of the country. I am not suggesting you ignore ticks. I am suggesting that a whole-picture risk assessment, combined with proactive and natural strategies, can substantially reduce your dog’s tick burden without the neurological, ecological, and pharmacological costs of monthly isoxazoline administration.

Step 1: Assess Your Actual Risk

Tick risk is highly regional and seasonal. A dog in a dense urban environment with manicured lawns faces very different risks than a dog who hikes in wooded, brushy terrain in Connecticut or the upper Midwest. Ask yourself:

• Does my dog spend time in wooded, brushy, or tall-grass environments?
• Am I in a high-prevalence Lyme disease region (Northeast, upper Midwest, Pacific Coast)?
• What tick species are present in my area, and what diseases do they carry?
• How thoroughly and consistently can I perform daily tick checks after outdoor time?
• Does my dog have any history of seizures, neurological sensitivity, or other conditions that increase their risk from isoxazolines?

Your answers should drive a targeted, individualized plan — not a one-size-fits-all monthly chew marketed to every dog owner regardless of geography, season, or risk.

Step 2: Tick Avoidance in the Environment

The single best tick prevention strategy is avoidance — reducing the number of ticks your dog encounters before they ever have a chance to attach. You can significantly reduce tick pressure in your own yard and on trails through smart environmental management.

Yard Management

Keep grass mowed short. Clear leaf litter and brush piles where ticks shelter. Create a wood chip or gravel border between your lawn and wooded areas — ticks rarely cross dry, open barriers. Learn more at TickEncounter.

Wildlife Deterrence

Deer and rodents are the primary tick hosts. Deer-resistant fencing and rodent habitat reduction (sealing brush piles, elevating woodpiles) can dramatically lower tick populations in your yard.

Guinea Fowl, Other Poultry

Where feasible, guinea fowl are voracious tick predators and can substantially reduce tick populations in a yard or property over a season. A time-honored, pesticide-free approach.

Trail Awareness

Ticks concentrate at the edges of trails, in brushy understory, and in tall grass. Walk dogs in the center of paths, avoid brushy margins, and steer clear of areas with dense deer activity.

Step 3: Natural Repellent Strategies

While I want to be honest that no natural product matches the killing efficacy of systemic isoxazolines, several natural approaches meaningfully reduce tick attachment and can be part of a layered protection strategy.

• Cedar oil sprays: Cedar oil is an effective natural acaricide and tick repellent. Products formulated specifically for dogs can be sprayed on the coat before outdoor activity, particularly around the ears, neck, and legs where ticks preferentially attach.
• Geraniol and rose geranium oil: Rose geranium essential oil has well-documented tick-repellent properties. A small amount applied to the collar or at attachment points before walks provides genuine repellency. Use properly diluted formulations and confirm safety for your specific dog.
• Lemon eucalyptus-based repellents: The Centers for Disease Control recognizes oil of lemon eucalyptus as an effective repellent for humans, and it shows promise for pets in appropriately formulated products.
• Apple cider vinegar in diet: While not a standalone solution, many holistic practitioners find that dogs who eat a fresh, whole-food diet with added apple cider vinegar are less attractive to parasites in general. A healthy, robust skin barrier is part of your dog’s natural defense.
• Diatomaceous earth: Food-grade diatomaceous earth can be lightly applied to the coat and bedding to deter fleas and ticks mechanically. It is non-toxic to mammals when used appropriately and not inhaled in excess.
• Tick-repellent collars using essential oils: Several natural collars use essential oil blends that provide genuine deterrence. These are not equivalent to Seresto or similar pesticide-impregnated collars, but they add a layer of protection with a far safer profile.

Step 4: The Daily Tick Check — Your Most Important Tool

I cannot overstate this: thorough daily tick checks are the most reliable and safest tick management tool available. A tick found and removed within a few hours of attachment — before it has fully engorged — carries minimal disease transmission risk for most pathogens. Make the tick check part of your end-of-walk routine. Run your fingers through the coat, paying special attention to the ears, neck, groin, armpits, between the toes, and around the tail base. Learn what an unengorged tick feels like — they are small and easy to miss.

A hair or lint roller can be very helpful in picking up even the smallest ticks. Go with and against the lay of the coat, including legs and under the abdomen. Don’t forget to yourself thoroughly as well.

Remove any attached tick promptly with your favorite tick removal tool, pulling straight upward with steady pressure. Save the tick in a sealed bag in case testing is needed. Note the date and location of attachment.

My Bottom Line

I understand why isoxazolines are popular. They are convenient,  heavily promoted, and for the majority of dogs, serious adverse reactions don’t occur. But “most dogs tolerate it” is not the same as “this is the right choice for your dog.” The neurological risk is real and documented by the FDA itself. The mechanism of action means your dog is carrying a systemic neurotoxic insecticide in their bloodstream for a month or more at a time. The combination products like Simparica Trio bundle in additional drugs your dog may not need year-round. The products don’t prevent tick attachment or the anxiety that comes with finding a tick on your dog. And now we know the drugs are contaminating our soil and potentially devastating insect populations through fecal excretion.

For most dogs and most households, a thoughtful, layered approach — environmental management, natural repellents, daily tick checks, and targeted use of safer single-ingredient preventatives for specific, documented risks — is both effective and far less burdensome to your dog’s health and your local ecosystem.

If you are in a high-risk tick disease area, have an outdoor-intensive dog, or are managing a specific medical situation, let’s talk. A one-on-one consultation allows us to look at your dog’s health history, your region’s specific risks, and build a plan that makes sense — not just a plan that’s easiest to dispense.

Your dog’s health is worth that extra layer of thought. That’s what we’re here for.