Hello,
  I just wanted to help give you more information on Frontline Plus for cats...it works great! I have used it for many years on my cat and it works however, they now seem to sell on ebay Frontline plus MADE for DOGS but selling it  for cats putting it in smaller doses...the problem?...Its NOT the same amount of Ingredients, active ingredients in both dogs and cats are not the same in the extra large dog size its 9.8 Fipronil and the other active ingredient is (s) methoprene which is 8.8....Now the Actual Frontline Plus for cats is 9.8 Fipronil (which is the same) but the other (s) methoprene is 11.8...this is the ingredient that keeps the eggs from developing...and sterilize the fleas and as you can see is higher in the cat product.....so if you use the dog product on the cats it will kill the fleas..but they will come back SOONER than if you purchased the Cat product, in the long run your not solving the problem of fleas:)..and if you give it more then recommended you could cause harm with to much Fipronil, I personally would not play with my cats health to save a couple of bucks...so just thought id give you more information to help you make a better informed decision ...and to know there is a difference, Merrill went through years of testing, they would have just made the ingredients for cats and dogs the same if they felt that would be the best product:)....... Happy Kitty tidings! Hope this guide was helpful!