I had been using cheapo products before buying this and at least one of the cats reacted badly every single time. I've decided that the manufacturer of Advantage definitely puts some of what they charge into making sure their product is safe. I'd prefer not to need anything like this but we do need it and it didn't make our cats sick at all. And it worked so quickly! About an hour after I applied it, both the cats were jumping around, I'm sure because the fleas were going nuts trying to get away from the poison. The cats had settled down quite a bit within about 5 hours and in the morning they were completely different kitties. No more scratching, biting, pacing around, meowing for no real reason, etc.  It's been several weeks since I used it on them and they are starting to have some trouble again but I think that's really because we got lazy and didn't treat one part of the apartment with diatomaceous earth when we did the rest about a month ago. I'll be using this every month from now on. It's worth the cost, especially at the prices we can get it for here at Amazon.