From: Dr. Kevin Francis DVM

Location: Trinidad

If you're reading this, then chances are  you’ve tried everything to protect your pet. I know exactly how you feel because I’ve seen it in my clinic for over 25 years.

.Back in 2005, I was just like you, prescribing monthly baths, expensive sprays, and seeing the same pets come back, still infested.

Then in the 2010's they introduced the Isoxazolines. These are the active ingredients behind popular tick and flea products like Bravecto® (fluralaner), NexGard® (afoxolaner), and Simparica® (sarolaner). AND WOW!! THEY WORKED!!! .nothing we saw up till then worked better, it was the proverbial better than sliced bread formulation..... until.....

My first doubts came not from data, but from what I saw with my own eyes.

I remember a small mixed-breed terrier, a happy, energetic dog named “Rusty.” His owner brought him in for a routine checkup, complaining about fleas. I prescribed a popular oral product, and within hours Rusty’s fleas were gone. But by the next morning, he was trembling uncontrollably. He couldn’t walk properly, his eyes darted in rapid movements, and his muscles spasmed in short, violent bursts.

We stabilized him with fluids and anticonvulsants, and he survived. But that moment stuck with me.

At first, I assumed it was an isolated case. But then, weeks later, a cat presented with severe lethargy and anorexia after taking another isoxazoline-based medication. Later, a Doberman developed seizures after a dose that had previously caused no reaction.

I began digging deeper. The U.S. Food and Drug Administration (FDA) had issued a warning in 2018 after reviewing thousands of adverse event reports submitted through its pharmacovigilance system. The FDA confirmed that the entire isoxazoline class was linked to neurological adverse effects, including tremors, ataxia, and seizures, even in pets with no prior neurological history (U.S. Food and Drug Administration [FDA], 2018).

The more I read, the more unsettled I became. The FDA report noted that while these drugs were effective at killing parasites, “adverse events involving neurologic reactions such as muscle tremors, ataxia, and seizures have been reported across all isoxazoline products.”

Further, internal safety data shared by European regulators showed even higher rates of severe outcomes. The EMA’s Committee for Medicinal Products for Veterinary Use (CVMP) documented post-market reports with fatality rates ranging from 13–23% in some datasets and seizure occurrence as high as 20.5% in others (EMA, 2019).

That was not the “one in a million” risk we’d been led to believe.

.So then: