I just returned from the animal emergency center after both of my cats got very sick after using the 4 in 1 flea medicine. They'll be there overnight with catheters in them. Thank God they are going to be okay but I was completely unaware of the possibilty of them becoming that sick. It is sick that this product is being sold on the shelves and its completely toxic. The doctor told me 1 in 3 cats have a reaction to flea medicine, but I had 2 cats and they both reacted. Maybe its just coincidence, but I think Hartz needs a kick in the ass.