Help! I’m scared to give my pet credelio.

Both my senior and mid aged cats are riddled with fleas I gave them a bath and it helped a bit it’s been about a month but there is still fleas but not nearly as much as it was …. I took my pet in for something unrelated and they gave me the flea meds for both pets and I’m terrified of giving it to them because of all those flea meds horror storys/seizures/overdose… I did give them the topical advantage almost a month ago but the vet said it was good to go on giving them this? Which I thought was weird. Plz ease my mind 😭🙏