Hold Off on Those Oral Flea-Killing Medications

The FDA has linked some flea control products to neurological damage in dogs and cats.


On September 20, the FDA issued a communication for pet owners and veterinarians, warning about the potential for neurologic adverse events following the administration of certain flea and tick products to dogs and cats. The products named in the release are oral products, available by veterinary prescription only, that contain isoxazoline-based ingredients. These include Bravecto, Credelio, Nexgard, and Simparica.

As we described in September 2017, both Nexgard and Simparica have verbiage on their package inserts that say, “Use with caution in dogs with a history of seizures or neurologic disorders.” To date, Bravecto has not included this recommendation on its product insert, though we extended this warning to dog owners in our article, based on the fact that the active ingredients in the products use the same mode of action. Credelio is a newer drug, given market approval by the FDA in January 2018; its package insert does include the statement “Use with caution in dogs with a history of seizures.”

nexguard flea control

The FDA communication notes, “In the first three years after approval, the FDA pays particularly close attention to adverse event reports, looking for any safety information that may emerge.” In the case of the isoxazoline-based flea-killing medications, the FDA observed a significant number of “adverse drug experience” reports mentioning these medications and pets that experienced seizures. The FDA has asked the drug manufacturers to add information about this issue to their product labels, and encourages veterinarians to “use their specialized training to review their patients’ medical histories and determine, in consultation with pet owners, whether a product in the isoxazoline class is appropriate for the pet.”

bravecto flea control

The communication did not include any mention of flea-control medications that contain spinosyn-based ingredients (Comfortis or AcuGuard, and Trifexis or ComboGuard), which work in a very similar fashion to the isoxazolines. However, the package insert for these medications include the statement “Use with caution in dogs with pre-existing epilepsy.”

In light of this FDA communication, Whole Dog Journal would like to make the following recommendations:

1. If your dog has ever had a seizure, has a known seizure disorder, or is descended from a breed or family that is prone to developing seizures:

Do not administer to your dog any flea-control product that contains isoxazoline-based ingredients (Bravecto, Credelio, Nexgard, and Simparica) – OR products that contain spinosyn-based ingredients (Comfortis or AcuGuard, and Trifexis or ComboGuard).

2. Until more information is made available by the FDA about the relative risk being posed by these products, we’d hold off on giving ANY dog ANY of these oral flea medications.

comfortis flea control

An exception could perhaps be made for dogs who have severe flea allergy, live in an area where fleas are a constant threat, who suffer serious adverse reactions to topical flea-killing pesticides, and who have taken these oral products previously without any adverse reactions. (Read carefully: That would encompass a very tiny pool of potential candidates.)

3. As we noted in our September 2017 article, oral products that kill fleas pose many disadvantages over topical flea-killing pesticides.

Most important is the fact that, once consumed, there is no way to reverse or neutralize their activity in case of an adverse reaction. In contrast, if a dog has an adverse reaction to a topical pesticide, the dog can be thoroughly washed and re-washed to reduce the product’s absorption and shorten its effect on the dog.

4. The FDA did not include information about what specifically prompted the publication of its communication.

Has there been a spike in the number of reported cases of seizures among dogs (and cats) treated with these drugs? Or are the numbers of seizures being reported far greater than the pre-market approval studies led the FDA to expect? We have asked the FDA for this information and will report further as we learn more.


  1. We gave our 15 lb 1.5 year old spaniel mix one Credelio tablet in spring for fleas.
    He began having tremors 2 weeks later! We thought he was in pain at first, but realized it had to be the Credelio. Will NEVER give it to him again! The scary part is it has to wear off.
    Nothing neutralizes this toxic medicine.

  2. Nice presented blog and very well written. My dog had fleas and ticks two years ago. My vet recommended Nexgard or Bravecto both treatments for the dog. I choose Nexgard because it comes in a tasty beef flavor that is easy to administer. And only single chew covers the whole month. After using the preventive, my dog was fleas and ticks free. I was happy for the amazing result. I would recommend this treatment. Thanks!

  3. We are very happy with this for our dog. We live in VA where we see a lot of ticks and since she’s been taking the chews, she doesn’t have any issues with ticks and other bugs on her. It’s worth the buy to give it to her over applying something over her coat or giving her a shot. best buy at budgtepetcare price is competently low.