Most contain pyrethrins or synthetic pyrethrin derivatives. They also have very short residual activity and may require frequent application. The product must get to the skin level to be very effective. Often, residual activity is less than optimal for animals that become dirty. Fleas do not have to feed on the pet to be affected.
Fleas are affected within minutes of contact with the crystal and death quickly follows.
Frequent bathing can remove this product from the hair coat. Intoxication and death for the flea occurs upon taking a blood meal. Because the products are within the skin, frequent bathing has minimal effect on duration of effect. Toxicities are not uncommon, even when used according to directions. Never use dog products on cats.