Frontline is a topical flea and tick treatment for dogs and cats. It is applied to the skin between the shoulder blades, and it works by killing fleas and ticks on contact. Frontline is generally safe for dogs, but it can be harmful if too much is applied. Applying too much Frontline can cause side effects such as vomiting, diarrhea, seizures, and even death. In addition, Frontline is toxic to cats, and can be fatal if applied to a cat.