Over-the-Counter flea treatments and collars found at drugstores and pet stores are not recommended. Veterinarians recommend not using them as they can be dangerous. There have been cat deaths related to the active ingredient – Pyrethrins. That is the active ingredient in most of those products. Pyrethrins are a derivative of a flower that is toxic to cats, chrysanthemums.
Some of the side effects associated with those over-the-counter products are vomiting, seizures and lethargy, and even death. Veterinarians recommend a product such as Advantage or Frontline. They will kill the fleas without typically harming the cat, as long as you use it correctly. Optimally you should order it from the veterinarian but some Petco and PetSmart stores sell it. True, it’s way more expensive. But trust me, it’s effective and worth it.
One thing to keep in mind that you will need to choose the right formulation for your cat’s weight. Don’t use a formula for larger cats on smaller ones and vice versa. That can cause issues or just not work well and get rid of the fleas. NEVER EVER use a formulation for dogs on your cats.