My vet recently switched to simpirico and I bought one dose. I chose to consider if I want to continue or switch to something else. After careful consideration, I've chosen not to continue with it even though there's been no side effects from the one dose. The FDA issued a warning for drugs in the isoxazoline class as they may cause neurological symptoms. My fear is neurological symptoms may not be apparent. Humans and dogs can have a seizure with no apparent signs or symptoms.
FDA fact sheet about potential neurologic adverse events in dogs and cats when treated with the flea and tick drugs Bravecto, Credelio, Nexgard, or Simparica.
For the record, the above is solely my opinion within the context of my own dog.
I would not give advise to use or not to use but leave it up to each owner what is the best thing to do for their dog. The medication is vet approved and dogs have taken it and been absolutely fine. For example, Credelio is in the same aforementioned class of drug and I gave it to my dog for a year with no apparent side effects. Granted I did not do independent research of the potential side effects, like I am now. In summary, all meds have potential side effects.
To clarify, the FDA didn't issue an actual warning, but rather an alert. This is because the drug label hadn't previously included some potential neurological side effects which reports were being made on. The manufacturers are to add these potential side effects on the warning label. I don't know if this has been done yet. The FDA considers the drugs safe but do recommend speaking with your vet about all potential side effects, especially in relation to your pets general health history.
The FDA's role is to collect data from vets, clinics, and owners and issue guidance based on that data.
This is the link to the FDA requesting label information changes by the manufacturers: