I've been using 'cat grass' for years. The one I use is made by a company called Cosmic and I think they call it 'kitty herbs' or something like that. It has Oat, Barley and Wheat seeds in it. I can get it at most pet stores. It's safe, and the only problems I've ever noticed is if a cat eats too much of it they might vomit occassionally.

It does help them to pass hairballs too because it's fiber for them. It helps satisfy their urge to chew on plants, and most of them like it. I even gave it to the kittens when they were only 3 1/2 months old, but their little kitten teeth have a more difficult time chewing a blade off than older kittens do.