Olney's Lesions might result from high doses of dissociatives, but they have not been demonstrated in humans.
http://en.wikipedia.org/wiki/Olney's_lesions

So 300mg of DXM twice a month shouldn't cause much of a problem. I've known people to ingest upwards of 400mg daily for weeks at a time and the result was a sort of delerium that faded with abstinence.
Nullific Reviewed by Nullific on . DXM Saftey Question Hey all, ive done DXM once and loved it, I've skimmed over erowid looking for saftey facts, because im concious about what i put in my body. how dangerous is dxm if used around 300mgs at maximum twice a month? any links would be appreciated so you all dont have to do much typing for me :) Rating: 5