It's pretty disgusting to think of worms living in your cat's gut. Worse than disgusting, though, worms can make your cat sick by preventing his body from absorbing nutrients. A regular worming program will help keep him healthy.
Until your kitty is about four months old, she'll need wormed every two weeks. Kittens pick up worms easily, they can be hard to get rid of and they can have serious health effects. Once your kitty starts growing up, she won't need wormed nearly as often. An outdoor cat that hunts and is exposed to fleas and ticks should be wormed every two to three months, while an indoor cat only needs to be wormed about every six months. Explain your cat's living situation to your vet for recommendations if you're not sure how often to worm your adult cat.
Worming medications that come in pill form aren't really ideal for cats because it's so hard to get a cat to take a pill. Unlike dogs, they aren't easily fooled by hiding a pill in food. If you have to use a pill worming medication, gently open your cat's mouth, place the pill at the back of her throat and then hold her mouth closed without blocking her nose. Stroke her throat to encourage her to swallow. If she spits the pill out, try again. It will probably take a couple tries to get the pill down. Cats are pretty smart when it comes to avoiding pills.
Paste worming medication isn't easy to give to cats, but it's a little easier than pills. Follow the manufacturer's dosing instructions, which usually include turning a knob on a syringe filled with paste according to your cat's weight. Gently hold your cat's mouth open and insert the syringe between his back teeth. Press the plunger and deposit the paste on the back of his tongue. Don't put it so far back that he chokes. Watch your cat to make sure he swallows most of the paste and doesn't spit it out. If he does spit most of it out, scoop it on to a popsicle stick and spread it on his tongue again.
External spot treatments are a dream to give to cats, especially if you've already had to struggle to try and get your cat to swallow a pill or paste. Simply follow the manufacturer's instructions for application, which usually involves placing a few drops of solution on the fur between your cat's shoulder blades. The downside of spot treatments for worms is that they are usually more expensive than pills and pastes.
Only use worming medication designed specifically for cats. Even de-wormer made for dogs and puppies is too strong for your cat. If your cat is sick, elderly or extremely underweight, check with your vet before treating him. You might need to adjust the dosage or wait until he's a little healthier. Finally, double-check the manufacturer's instructions for kittens. Some de-wormers are made only for adult cats and can make young kittens very sick.