I once knew a cat called Baby. Its owner had adopted it in Japan while she lived there, and brought it back to the US at great expense. The cat survived the trip just fine. Later he was caught by a dog and severely mauled, but his owner (again at great expense) got him all fixed back up. Later, she went away for the weekend. While she was away, her housemate decided to use some grocery-store medicine on the cat. The cat went into seizures and died. But the medication was a real bargain.
My wife’s worked as a vet tech, and is now halfway through vet school. She hates over-the-counter medications with a passion because she’s seen too many animals come into the vet dead-on-arrival from OTC medications. She says “People think that vets want you to use prescription medication so the vet makes more money. That’s not true; vets want you to use vet-prescribed medication because the vet cares about the health of the animal.”
As far as flea control goes, the cats in the Sengkelat household used Advantage, once a month, without fail. Five years, no fleas, and that includes when they used to be indoor-outdoor cats. We can’t get Advantage where we live now, so we use Revolution.
Advantage was great 'cause it was toxic to fleas for a month, but totally safe for the cats. The spiky-hair look only lasted a day or two. And any fleas that jumped on the cat would DIE, so even if a flea got into the house, it’d find the cat and that’d be the end of the flea.
Someone back there recommended Program. Program isn’t nearly as effective as Advantage, as Program only really works if you aren’t getting an influx of new fleas.
Also, with cats full of fleas, they’re at greater risk for tapeworms and heartworms. Tapeworms are gross (and you can get them too, eeeeew) but heartworms can kill your cat. Even killing the hearworms, if they’re well established, can kill your cat. See your vet.
I’m glad so many other folks have already told you not to use the OTC products. It can’t hurt to hear it a million times; they’re just bad, bad, bad.