Med reports · Drug class
The isoxazoline flea and tick meds.
Bravecto, NexGard, Simparica and Credelio are marketed as rivals, but underneath they are cousins. They belong to one family of chemicals called the isoxazolines, they work the same way, and in 2018 the FDA issued one advisory covering all of them. If you have been searching one brand name, the thing you actually want is this page.
A report is not a rate. These are reports people filed with the FDA, not verified findings. Anyone can file one. And a count goes up with how many dogs take a med, not just how often something goes wrong, so a product given to millions will always show more reports than a rare one.
Nothing on this page can tell you how likely a side effect is for your dog. Nobody can calculate that from this data, because it records the reports and not the number of dogs treated. Use it to see what has been reported and what to keep an eye on, then talk to your vet. How to read these reports.
The advisory
What the FDA actually said
This is the part that gets mangled everywhere else, so here it is straight. The FDA did not warn owners off these drugs. It did the opposite and then added a caveat. In its own words:
“The FDA considers products in the isoxazoline class to be safe and effective for dogs and cats but is providing this information so that pet owners and veterinarians can take it into consideration when choosing flea and tick products for their pets.”
The caveat is real and worth knowing. The FDA says these products “have been associated with neurologic adverse reactions, including muscle tremors, ataxia, and seizures in some dogs and cats”, and it notes that seizures can happen in a dog with no history of them. Ataxia is the medical word for wobbliness, a dog walking like the floor is moving.
So both things are true at once, and most coverage picks whichever half suits it. The class is approved and the FDA stands behind it. It also wants you and your vet to have the seizure information before you choose. That is a smaller, more useful statement than either “perfectly safe” or “the FDA warned about this drug”.
Read the FDA advisory in full. It is short, it is written for owners rather than vets, and it was last updated on 8 August 2023.
The family
Which products are in this class
FDA’s own list, for products given to dogs. Note how many of these are the same brand name on different products, which is the single most common way people end up reading about the wrong one.
| Product | Active ingredient | Form |
|---|---|---|
| Bravecto | fluralaner | tablets for dogs |
| Bravecto | fluralaner | topical solution for cats and dogs |
| Bravecto 1-month | fluralaner | tablets for dogs |
| Credelio | lotilaner | tablets for dogs and cats |
| NexGard | afoxolaner | tablets for dogs |
| NexGard Plus | afoxolaner, moxidectin, pyrantel | chewable tablets for dogs |
| Simparica | sarolaner | tablets for dogs |
| Simparica Trio | sarolaner, moxidectin, pyrantel | tablets for dogs |
The advisory also covers three cat-only products, Bravecto Plus, NexGard Combo and Revolution Plus, which are outside the scope of this page.
“Bravecto” alone is four different products. So if you are checking on something your vet dispensed, check the form and the duration on the box, not just the name. Our Bravecto chewable tablets for dogs page covers exactly one of those four.
Reading the data
Why you cannot compare these by report count
The obvious thing to want, once you know these are cousins, is a table ranking them. Which one has the fewest seizure reports? We are not going to build that, and it is worth explaining why rather than just refusing.
The FDA record counts reports. It does not count doses. NexGard and Bravecto between them have sold enormous volumes over a decade; a newer or smaller product has not. Put their report counts side by side and you have mostly drawn a chart of market share with a scary title on it. Any ranking built this way would punish the popular product for being popular, and a reader would take a real decision from a number that means nothing.
What you can do honestly is read one product at a time, in the open, with its limits stated. That is what the individual pages do.
One class-wide finding
A lot of these reports are not about harm
Something surprising sits in this data once you separate it out. A large share of what gets reported to the FDA about parasite medications is not an injury at all. It is the product not working: the fleas came back, the ticks were still there. Treatment failure is a reportable event, so it lands in the same dataset as a seizure.
How large a share depends heavily on the product, which is why we check it per page rather than repeating a number. On the Bravecto chewable record it is 19% of all reported terms. On the heartworm preventives it is a much bigger slice than that, big enough that treatment failure dominates their top reported terms outright.
It matters for you specifically because it changes what a big number means. A product with a lot of reports is not necessarily a product hurting a lot of dogs. Sometimes it is a product a lot of people felt let down by. Those are genuinely different problems and only one of them is a safety question.
What to do with this
If your dog has a seizure history
That is the conversation the FDA is explicitly asking you to have. Its advisory tells owners to talk to a vet about the dog’s medical history before choosing a product in this class. If your dog has ever had a seizure, or a neurological diagnosis, that is information your vet needs and it may change what they reach for.
One thing worth taking into that conversation: skipping prevention is not a neutral choice. Fleas and ticks carry real disease, so the decision is rarely between a med and nothing.
If something already happened, call your vet, then consider adding your own report to the record these pages are built from.