For many years Frontline has been used successfully in many countries, and has often been the first choice of vets. It comes in a number of versions, and you need the one that is right for your cat, including its weight. The problem with it recently in some places has been a result of its success - as with all medicines, it is meeting a super-resistant problem, in this case the fleas are becoming resistant to it, and so it doesn't work any more. My brother lives in the south of the UK and vets have stopped using it there, because of this problem. So it depends a lot on where you live whether you should use it, and the only way to find out is to ask the vet.