Adverse reactions to oral flea and tick prevention in the news

You may have heard news stories in the past day or so about adverse reactions to oral flea and tick preventions for dogs and cats in the isoxazoline class of medications.  This class of medication includes the medications Bravecto, Credelio, Nexgard, and Simparica.  We routinely recommend using Nexgard and Simparica for our patients and continue to believe that the medications in this class of drugs are safe for the majority of pets so we wanted to go over a few things about these medications to hopefully address concerns you may have about using them.
All animal drugs that are approved by the FDA go through an extensive amount of safety testing prior to their approval.  This includes testing safety of intentional overdoses of many times the labeled dose.  In addition, manufacturers are required to report any potential or suspected adverse reactions after a product has been approved.   The most frequently reported adverse reaction to this class of drugs is gastrointestinal upset (vomiting or diarrhea).  As veterinarians, if we see a pet that we suspect may have had an adverse reaction to a drug, we report it to the manufacturer, who then reports it to the FDA.  The FDA monitors these reports and if they feel a product is unsafe, they will revoke its approval.  The FDA made this announcement to help pet owners and veterinarians be more aware of the potential side effects of this class of medications, not because they think the medication is unsafe.  In fact, the potential for neurologic side effects is already listed as a known possible adverse reaction on the label.
We talked with Zoetis (the manufacturer of Simparica) after the FDA’s announcement to get some concrete numbers on the frequency of these adverse reactions.  Currently, for Simparica, less than 0.01% of dogs taking this medication have experienced any neurologic side effects.  The FDA considers this level to be “very rare”.
The isoxazoline class of drugs acts at receptors in the central nervous system of invertebrates like fleas and ticks.  Dogs and cats have a barrier that protects these compounds from getting into their central nervous system (the blood-brain barrier).  A very small percentage of dogs can have a mutation that allows some drug to cross this barrier, but even in those cases the receptor site that these drugs act on is rare to absent in most dogs.  This mechanism has been known since prior to the approval of these products.
This announcement from the FDA also serves as a good reminder to obtain medications from your veterinarian or from a pharmacy associated with your veterinarian (such as Vets First Choice).  Manufacturers do not sell to online pharmacies that are not associated with a veterinary practice.  This means these online companies are not getting medications from the manufacturers but from another, unknown and unregulated source.  The products you receive from an online pharmacy that is not associated with a veterinarian may be counterfeit, expired, or repackaged medications and are not backed by the manufacturers of these drugs in the event of an adverse reaction or disease (such as heartworm or lyme).
In summary, we continue to support the use of these medications to prevent fleas and ticks and their associated diseases.  We all use this class of medication in our own dogs for flea and tick prevention.  We are always happy to discuss any medications your pet is taking, including flea and tick preventatives.
To read the fact sheet published by the FDA for pet owners and veterinarians, please see the link below:
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The Doctors of Muddy Branch Veterinary Center

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