If you go into any search engine and type the word "Hartz" you'll have more info than you can stand! It has caused deaths in many cats, topicals (drops like Advantage) and the spray forms as well. I'd stick with the safe stuff like Advantage and Revolution which many places now carry without a prescription. If you type those words into the search engine, you may find some good online sources, but beware of the fake stuff that's being sold as Advantage etc. Better to just get it at your vets office.