We have the SAME exact situation in Alabama!!! Someone bought the Hartz drops and their cats almost died! There is an ingrediant that SOME cats are allergic to or causes BIG side effects.
This lady was on the news here locally about a month ago. She contacted the company herself. They gave her the same spiel you got. They have NO plans of removing it from the market.
I worked in a Pet Store and I would NEVER recommend Hartz brand unless it was a toy. Even the toys I was picky on. I've used Hartz products a majority of my young life. Now as an Adult, I question it's quality and healthfulness. It scares me to see the number of Pet companies that put things out that are completely unsafe for animals.
Keep yourself educated and protected. No one else is going to do it for you!!!