On September 20th, the FDA came out with a public Alert regarding the potential for neurological adverse events related to flea/tick preventatives within the isoxazoline class – essentially all of the currently available oral flea/tick preventative products (NexGard, Bravecto, Simperica and Credelio).

There are still a lot of unanswered questions presented by the FDA’s Alert, but we are actively investigating this issue and will keep you posted!

To read the FDA Alert, you can follow the following link:


The FDA also has an additional Fact Sheet here:


Why is this Alert significant?

Over the past 3 years, since the release of these different products, the FDA has been receiving reports of neurological adverse events (muscle tremors, ataxia and seizures) believed to be related to the administration of drugs in the isoxazoline class.

This is significant, because these types of adverse events were not encountered during the safety trials performed with these products prior to their release.

Why weren’t these adverse events found during pre-release safety trials?

Before any pharmaceutical – medication or vaccine – can be released for use in the general population, it goes through several stages of safety trials, during which a sample group of individuals are evaluated for adverse side effects or events during the trial duration.  Anything abnormal that happens is reported as a potential adverse effect or event.  These events are often compared to a placebo group – who don’t receive the pharmaceutical – to factor for adverse effects or events that may not be related to the pharmaceutical.  For example, it is common with pharmaceutical trials to have a small percentage of individuals who demonstrate vomiting and/or diarrhea, whether they are in the “trial” group or the placebo group.

It is important to recognize that safety trials are done on a ‘sample size’ quantity of individuals – whether the sample size is a dozen or a hundred, it will pale in comparison to the amount of data gathered once a product is released and used in thousands to hundreds of thousands of individuals.  This is why the FDA mandates pharmaceutical companies collect and report data about suspected adverse effects or events that pet owners or their veterinarians report to them:

“The FDA monitors adverse drug event reports received from the public or veterinarians, other publicly available information (such a peer-reviewed scientific articles), and mandatory reports from the animal drug sponsor (the company that owns the right to market the drug).”

This allows for rare or uncommon side effects or adverse events to be detected, reported and monitored – these may not be detected in a smaller sample size of individuals.  This also allows for safety of a pharmaceutical to be evaluated in harder to anticipate situations, such as an individual with multiple illnesses or one with a slight difference in biology that responds differently from the rest of the population.

What are we doing about this as your veterinarian?

We are reaching out to the FDA to obtain the information our patients and pet owners will need to know regarding these new findings.  This way we can best answer your questions and give you the best possible advice.

What questions remain unanswered?

Unfortunately, the Alert from the FDA was not very specific.  Some questions we are working to get answers for include:

  • How many – or what percentage – of pet(s) were demonstrating these adverse event(s), and what were they specifically presenting with (i.e. what is the real risk)?
  • When were these event(s) occurring? Was it within a specific time frame of a first dose, or after several doses were given?
  • Were there any other factor(s) that affected risk of these adverse event(s)?
  • Did discontinuation of the medication(s) in adversely affected individuals result in resolution – or disappearance – of symptom(s)?

We will keep you posted as we continue to get more information for you and your pets!