It is not used widely as a pain killer. I'm curious how you came up with that claim?
It is used on animals. It is used on childeren. It is used on the elderly. This is because of its mild nature. The doses one takes in a "recreational" setting are much lower than if you took your 8 year old in for surgery.
The side effects you speak of were reported by William E White in 1998. He makes some rather great assumptions in coming to these conclusions. I've linked to Erowids response below. You cannot state for a fact that his paper is accurate because of these assumptions. I cannot state for a fact that he is wrong, but the evidence suggests something else is going on. EDIT: Your line should read, "There is currently no way to determine if humans are susceptable to this at all."
How often you do it is also a factor. Do you give your brain a break? Do you slip into an addicted binge for weeks or months? Responsibility is what keeps you safe. Hell, these rules apply to anything. You could drink water till you died.
During my time with K, I'd usual dose once then wait a couple weeks before doing it again. This went on for I'd say, 3 months. I've never experienced any of the side effects mentioned in these papers.
http://www.erowid.org/chemicals/dxm/dxm_health2.shtml
sillygirl: It's good that you don't try them. Without respect for drugs, they will destroy you. Just like a loaded gun. EDIT: Ever take aspirin? A lortab? Maybe some caffiene? What do you think those are? I'd learn to not fear drugs. I would research, and respect.