First off, I loved that Advantage offered different strengths for different sized cats. That alone drew me in because I have two cats, and they are two very different sizes. Second, I had read rave reviews about this product, although I was still kind of cautious because I had tried a prior product that had just as awesome reviews and it didn't work for me like it had for others. It was expensive to say the least, especially since I had to buy two separate boxes, and I went ahead and got the 6 dose boxes as well, because it was my last resort; I had tried a lot of others with no luck. Needless to say, this brand was the winner! Neither of my cats had reactions to this, and within only a couple of days, I noticed a huge difference. I've been using it now for 3 months, and I haven't seen even one flea on either of my cats, or in my house. Love this and will never, ever go back to any other brand! Quality of Ingredients It works like a charm with no allergic reactions. Side Effects My cats didn't have any, but I guess that doesn't mean that no cat ever will.