I have used Advantage for years and appreciated how it kept fleas at bay, although I have also been uncomfortable with the idea that I am pouring poison on the back of their necks.  I have two cats and only need to use the product every 3 months or so.  I'm not certain if they changed their formula or if it's because my cat is getting frail and old (she's 18 y.o.), but the last time I used this product on her she became so severely ill that she could barely move and I thought she was going to die.  I have never seen her so sick and it was heartbreaking to witness.  After a 24 hour period she slowly recovered, thankfully.  I read some other reviews and it looks like others have had this problem and one person said her cat DID die.  I don't like not being made aware of the possibility that I could be killing them by using this product!  As I mentioned, I had been using this product for years so I don't know why the sudden difference in response.  I just wanted to alert others of the possibility that their cat could have a severe reaction -- something the manufacturer should be doing!!  Unfortunately, my cat now has fleas and I am trying all that I can to remedy the situation, but nothing works as well as these poisons.