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What you need to know about isoxazolines.

To our Atlantic Vet Family:

FDA Announcement:

This past Friday, the FDA published an animal drug safety communication ( about potential adverse effects associated with a class of chemicals called isoxazolines. Isoxazolines are a synthetic chemical class that has broad spectrum insecticidal effects against a number of parasites such as fleas and ticks.

Most available products, such as Nexgard (Merial), Bravecto (Merck) and Simparica (Zoetis) are for oral administration in dogs. We do often recommend Bravecto topical for cats which is also an isooxazoline. Isoxazolines are rapidly absorbed in the blood and distributed throughout the body of the dog or cat. Once ingested by a parasite, isoxazolines cause paralysis and death.

The focus of the communication was regarding the occurrence of neurological adverse reactions such as seizures, muscle tremors, and ataxia (unsteadiness, loss of coordination) in some dogs and cats. The FDA is asking all manufacturers to communicate these risks more clearly through their labeling to both consumers and veterinarians.

Impact of Announcement on the Atlantic Vet Family: LOW

Atlantic Veterinary Hospital continues to recommend Simparica for dog over the age of 6 months, Nexgard for dogs under the age of 6 months and Bravecto for cats and dogs over the age of 6 months.

Our recommendation is based on the following:

  • This FDA safety communication does not provide any new information or new findings on the isoxazoline class of flea & tick preventatives. The potential neurological effects have been observed by each manufacturer during their studies.

  • Simparica

  • The top three adverse events that have been observed in pets taking Simparica are vomiting, diarrhea and lethargy and the occurrence of these side effects are 1 in 10,000.

  • The therapeutic dose of the isoxazoline in Simparica is between 2 to 4 mg/kg. Abnormal neurological effects were observed at dosages between 12 to 24 mg/kg, 3 to 5 times the suggested dosing.

  • Nexgard

  • The top three adverse events that have been observed in pets taking Nexgard are vomiting, itching and diarrhea. During the safety studies the maximum exposure dose (6.3 mg/kg) was administered three times a day for 21 days and no clinical signs were noted.

  • Bravecto

  • 1 in every 10,000 reported adverse side effects involve neurologic reactions.

  • Of all the reported adverse side effects in dogs the top three were vomiting, lethargy and diarrhea. The top three reported side effects in cats were vomiting, puritis and diarrhea.

Since we began recommending oral flea & tick preventatives, we have had no adverse neurological side effect among our patients.

The risks of not administering monthly flea & tick preventatives are significantly more impactful to a dog and their family. At Atlantic Vet, cases of lyme disease, flea infestations, and anaplasmosis, by far exceed even the least adverse side effects of these preventatives. Killing ticks through preventatives also prevents human family members from tick-borne illnesses such as lyme and anaplasmosis.

Atlantic Veterinary Hospital does not recommend these preventatives to dogs with known neurological problems.

If you have any additional questions or concerns regarding these products and your dog or cat, please do not hesitate to contact us either by email at or by phone at (781) 631-8881.

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