Flea "season" is fast approaching as the weather rapidly warms up, so now is a good time to cover flea control and flea product safety. FDA has an excellent page about the safe use of antiparasitic products in pets, which I encourage you to read even if your cat(s) are 100% indoors. Too often fleas manage to get indoors and be noticed by an owner on the weekend when the vet is not available to discuss appropriate control. The result can be a bad decision on the owner's part, and a sick or dead cat.
The most important take-home message is: NEVER use a product labeled for use in dogs only on a cat. You'd think I wouldn't have to say this, but it is a very common error, usually due to the "somebody said" phenomenon, whereby somebody who is NOT a veterinarian manages to convince a naive pet owner that they can save money by just buying dog products and using a lower dose on a cat. Big mistake.
I still highly recommend Advantage and Frontline as the safest and most effective OTC flea control products in cats. Advantage is now available at pet stores and online, so we will no longer carry it (we can't compete with the big boys on price and it's very expensive to stock it). My preference is the broader-spectrum prescription-only product Advantage Multi, which controls fleas, ear mites, heartworm, roundworms, and hookworms. It also in my experience controls mites other than ear mites, but has no label approval for those. If your cat goes outside on a regular basis, Advantage Multi is the way to go because the danger of those other parasites is very real.