You don't need a prescription for it. My mom uses it...she's a geezer (that's a term of endearment). She takes it because it's supposed to help with arthritis. That's another reason why us post-op knee people should be taking it...once you tear your acl or have some serious trauma to your knee, your chances of having arthritis in that joint go way up.
I personally don't take it, although I'd like to. I don't remember the reasons, but I was told that glucosamine is not good for younger people. I would recommend doing research on the product, just for peace of mind and to inform yourself...I know I still need to do that.