Capstar kills adult fleas on small dogs and cats from 30 minutes after administration. Because it stops fleas biting quickly, Capstar may be used to help control flea allergy dermatitis. Flea allergy dermatitis is a condition where a dog or cat becomes hypersensitive to flea saliva, which is injected into the skin as the flea feeds. Even small numbers of flea bites can cause intense itching and skin inflammation in a dog or cat with an allergy to flea saliva, so you may not even see fleas or flea dirt in their coat. In dogs, flea allergy dermatitis is most commonly seen as intense itchiness, hairloss and reddening of skin, particularly around the base of the tail.