Simparica Trio Safety: What to Know About Neurologic Side Effects

Simparica Trio has been in the news twice this year — once for an emergency authorization, and once for the safety questions that follow any widely used parasite drug. Both stories trace back to the same ingredient, and both are worth understanding before your dog’s next monthly dose.

Simparica Trio is a prescription chewable tablet from Zoetis that combines three active ingredients — sarolaner, moxidectin, and pyrantel — into one monthly dose. It was approved by the FDA in 2020 to prevent heartworm disease, treat and control fleas and several tick species, and treat and control roundworms and hookworms in dogs and puppies 8 weeks of age and older. It is available only by prescription from a licensed veterinarian — and the reason for that restriction is directly tied to the safety profile below.

The Emergency Authorization — and Its Limits

On August 13, 2026, the FDA issued an Emergency Use Authorization (EUA) allowing Simparica Trio to be used to treat New World screwworm infestations (myiasis) in dogs and puppies. Screwworm larvae feed on the living tissue of open wounds and can cause severe damage or death if untreated. Two limits matter for owners:

• The authorization covers treatment, not prevention. A dog already taking Simparica Trio for fleas and heartworm is not automatically protected against screwworm.
• The FDA stresses that most U.S. dogs currently face a low risk — confirmed detections remain limited to specific areas of Texas and New Mexico.

“We want dog owners to be informed, not alarmed,” said Timothy Schell, Ph.D., director of the FDA’s Center for Veterinary Medicine, advising owners to know the signs of infestation — visible larvae or a foul odor from a wound or body opening — and to see a veterinarian immediately if they suspect one.

The Safety Issue: Neurologic Adverse Events

Sarolaner belongs to the isoxazoline class of antiparasitic drugs, the same class as several other popular flea and tick products. The FDA has been explicit about this class for years: isoxazoline products are safe and effective for most dogs, but they have been associated with neurologic adverse reactions, including:

• Muscle tremors
• Ataxia — loss of coordination, wobbling, or stumbling
• Seizures

Those reactions have been reported even in dogs with no history of seizures. For that reason, the FDA advises caution when isoxazoline products are used in dogs with a history of seizures or other neurologic disorders — and it is part of why professional expertise is required to prescribe and monitor these drugs. That puts Simparica Trio in the same regulatory conversation as other FDA actions on pet products this year, from the agency’s Raaw Energy dog food warning to this month’s authorizations: the products are useful, but they are not casual purchases.

Warning Signs to Watch After a Dose

If your dog takes Simparica Trio, watch most closely in the day or two after each monthly chew, especially the first dose:

• Shaking, trembling, or muscle twitching that isn’t cold or excitement
• Unsteady walking, stumbling, or sudden clumsiness
• Disorientation or unusual lethargy
• A seizure — collapse, paddling legs, drooling, or loss of bladder control

If you see a seizure or severe symptoms, treat it as an emergency and contact a veterinarian right away. Note when the dose was given and when symptoms started — that timing is the single most useful detail you can give the vet. Do not give another dose until your veterinarian advises it.

Should Your Dog Stay on Simparica Trio?

For most dogs, regulators and veterinarians still consider the benefits — protection against heartworm disease, fleas, ticks, and Lyme-carrying deer ticks — to outweigh the risks, and the FDA has kept the whole isoxazoline class on the market with warnings rather than restrictions. The calculus changes if your dog:

• Has ever had a seizure or a neurologic disorder — tell your vet before the first dose, not after a reaction
• Has reacted to another isoxazoline product (plain Simparica, Bravecto, NexGard, or Credelio share the class)
• Is very small, very young, or on other medications your vet should factor in

Alternatives exist in other drug classes and in topical products, so a dog that doesn’t tolerate an isoxazoline is not out of options — but don’t simply stop parasite prevention without a replacement plan, because heartworm disease is far more dangerous than the reaction risk for most dogs. If your dog does experience a reaction, report it: adverse events can be reported to the FDA and to Zoetis, and those reports are what drive the label updates and fact sheets that keep the guidance current.

Source: FDA press announcement on the Simparica Trio Emergency Use Authorization, August 13, 2026; FDA fact sheet on isoxazoline-class flea and tick products