Comfortis can make existing seizure conditions worse, but in the studies they have not found it to cause seizures in healthy animals (and to cause them in animals with existing conditions, they had to over dose them). Vets won’t give it if the animal has a history of seizures though. My mom uses it on her dogs and cats – five dogs, seven cats and no side effects. Vets still prescribe it often enough for healthy animals here.
Each of the flea medications do have risks though. Advantage caused very bad frothing at the mouth and vomiting in one of our dogs. Sentinel caused vomiting and lethargy. It is really scary when they have a reaction to them.
Many of them are designed to stop the flea biting rather than kill. It allows egg laying and reinfestation from a yard, or anywhere else fleas stay. Which is why sometimes it seems like they do not work, they just keep picking up new fleas that then have to wait days to die off and it gives them time to pick up more new fleas. It is a vicious cycle to try to break. My dog was allergic to fleas and would get horrible scabs all over her body if she even had a couple. I hated those things! They can be a pain to get rid of.