We treated our two cats, ages 1 and 4, with the flea control drops. Both cats were shivering and tremoring within 12 hours. I had no idea what may have caused it and then I found out from our vet that Hartz Flea control products are basically PESTICIDES! He said that what was contained in the products was the same stuff you put on your yard to kill grubs, etc. After bathing the cats to get the poison off of them and some IV fluids, they came out of it. Thank goodness! Our vet said that the longer the product stays on the skin, the worse it becomes. We have used Frontline products for years and tried a "quick" remedy this time. It turned out to be a costly lesson. NEVER AGAIN!!!