Last year, my cats had a flea problem, and I had tried everything under the sun to solve this problem. When over-the-counter remedies failed me (big time), I went to my vet and purchased a pack of Advantage Flea Control Topical Solution. I wish I had done it sooner, it would have saved me some $$, not to mention bringing relief to my kitties much quicker.
Advantage comes in small plastic capsules. There are four of them in the pack, and one capsule, applied to the back of the cat's neck, is said to work for a month. When I carefully applied the product, following the directions found in the box as well as my vet's instructions, I could see it work within minutes: my cats stopped scratching and looked very comfortable. Advantage not only kills adult fleas (98-100% of them within 12 hours!) but also eliminates flea eggs and larvae, this breaking the flea life cycle.
Our flea problem was completely solved, within a day. My cats have stayed flea-free ever since, months and months later. Advantage is more expensive than regular brands, but I highly recommend it; it works 100%. You will spend more trying this and that, like I did, and getting only partial results, if any at all.