Is Flouride bad for us?

I remember a friend of mine who was heavily into natural health went on and on about how bad regualar toothpaste was because of the flouride. I never really thought about it much but started to buy natural toothpaste from the health food store, now I am just accustomed to it, but when people ask me why I am never quite sure what to tell them. I know it is because of the flouride but I can't remember why. I do know one thing, when I use regular toothpaste it will make my stomach do loop-de-loops. So I stick the the natural toothpaste.