Posted by: anubee on: November 28, 2009
Frontline Spot and Frontline Plus, are both largely safe for all animals, including puppies older than eight weeks, or animals that are pregnant or nursing. However, in rare instances, some pets may develop sensitivity to the product, manifested by irritation at the application spot. In most cases this is harmless, but if the irritation continues for more than a week or increases, you should contact your vet.
One common concern that some Frontline users have is the appearance of more fleas directly after treatment. This happens because the Frontline causes the existing fleas to move away from the surface of the animal and towards the tip of the coat. This makes them more easily seen by the naked eye.
Frontline users should also know that the product is waterproof. Since the product is stored in your pet’s oil glands, it continues to be distributed after you wash your pet.
Article Source: http://www.ehow.com/how-does_4680280_frontline-work.html