On Letterman last night, Charlie Sheen was asked whether crack or booze had been the reason for his 2011 meltdown. Charlie said that it had not been, but wished it was because what happened was much more dark but would not reveal what it was, except to say he had to do what was right.
Source:
http://www.crazydaysandnights.net/2013/01/charlie-sheen-says-crack-and-booze-not.html