DXM is a dissociative drug commonly found in OTC cough syrups as a cough-supressant agent. When taken at high enough doses, it basically fucks you up big time. I love it, though I cannot recommend it to anyone who doesn't know as much as possible about it.

http://www.erowid.org/chemicals/dxm/dxm.shtml

My official stance: don't do it.
My real stance: Research it as much as possible, learn everything you can about it, and decide for yourself. If you do that, you'll find that it's actually quite safe (though using it too much IS very very bad and my REAL stance on that is NEVER use it more than on a monthly basis... but everyone has their own choice).