The best product to kill fleas is generally a spot treatment prescribed by veterinarians. Some of the products available in pet stores are potentially dangerous, and most are not very effective. Additionally there are some effective natural flea treatments for both controlling and killing the pesky creatures.
Veterinarians prescribe spot medications like Advantage and Revolution, which contain imidacloprid or fipronil. These are applied between a cat's shoulder blades and the base of their skull, where the cat cannot lick it off. It permeates the top layer of skin and, while it does not prevent fleas from jumping on a pet, quickly kills them when they do. They are very effective and generally safe when used as directed. Over-the-counter products like Bio Spot and Precor do not immediately kill fleas but render them infertile. The ingredients used in many over-the-counter formulations have raised safety and health concerns, and often the dosage used is too low to be truly effective on a cat. Some natural remedies use essential oils like eucalyptus and garlic oil, which can work but must be used with care since many cats can be sickened by essential oils. Diatomaceous earth is a very safe, natural product that kills fleas and has no harmful side effects for pets.
Some cats are not particularly bothered by fleas and will not scratch. If your cat goes outside or comes in contact with animals that do, periodically check for fleas. Usually the signs will be most visible on the belly and inner thighs. Look for little black dirt-like specks, which is dried blood left behind from feeding. Fleas are usually between 1/16 and 1/8 of an inch long, dark colored and will scurry through the fur. If you find evidence of fleas on your cat, be aware that most likely all animals in the household will have fleas and should be treated before it becomes an infestation.
The fastest and surest way to kill fleas is by getting one of the spot treatments from your veterinarian. These are very effective and, if you have a good relationship with your vet, should not require an office visit or the related cost. Tell her you found fleas on your pet and see if you can just pick up the treatment. Follow directions on the packaging.
Never be tempted to use canine flea treatments on a cat without checking with your vet first. While a couple are safe, some are lethal to cats and will kill them quite quickly. Do not use flea treatments on pregnant or nursing cats, kittens under 8 weeks of age or ill cats.
If using a vet-prescribed flea treatment is not an option, bathing your cat in mild dishwashing soap is very safe, although your cat won't like it at all. You may have to wrap your cat in a small towel first to contain her claws. Wet her thoroughly and lather well with the soap. Let it stay on the skin for several minutes before washing it off. Make sure to rinse thoroughly. Follow up with a good flea combing to remove dead or stunned fleas.
Diatomaceous earth (DE) is a natural, non-toxic dust made by grinding the shells of algae. The microscopic particles are sharp edged and kill fleas by cutting into their hard outer bodies. This method is very effective and safe, althoughit works a little bit slower than vet-prescribed spot treatments. It is safe for kittens and pregnant or nursing cats. It important to get food-grade DE, which is sold online or in health food and some feed and pet stores. Do not use the DE that is sold for filtering pool water, as it contains inedible and poisonous additives.