Many flea shampoos have pyrethrin as an active ingredient. Pyrethrin is toxic to cats and some dogs also have adverse reactions to pyrethrin. Depending on your circumstance you may want to avoid shampoos that contain pyrethrin.
Drown the Fleas
Many flea shampoos simply drown the fleas by reducing the water tension and this allows the water to fill the fleas airways. In the past I have used Dawn Dishwashing Liquid with rapid results. Dawn does have a side effect of drying out the dog or cats skin and this tends to lead to itching and scratching.
Soothe the Flea Bites
Dogs and cats that are infested with fleas also have many bites all over their bodies. With dogs I have found using a gentle peppermint tea tree oil shampoo helps soothe their flea bitten skin. Although most people don’t relish bathing their cat they can be safely bathed in a gentle cleansing shampoo that does not contain essential oils. Many essential oils are toxic to cats.