I do work for Merial, although we just got bought by a German company so our name will change soon. Frontline works okay at best. It does kill fleas, but it kills them really fucking slow. In a flea infestation, it typically takes at least 3 months until it is under control, and It is not fun to see fleas on your pet. Nexgard kills them so quick that you'll rarely see them. With Frontline, you'll see fleas. That can be a long 3+ months. I don't recommend topicals at all for flea infestations, because too much can go wrong. People mess up the application, people give their pets too many baths, etc. Comfortis is an oral product that you can give cats. I'd recommend that in the future for your cats. Feel free to ask any other questions on products. I'd be happy to give non-biased feedback.