One of the most popular "Iron Man" comic mini-series was the 'Demon in a Bottle' storyline involving Tony Stark's alcoholism.
Director Shane Black was reportedly interested in dealing with the issue in "Iron Man 3," but Marvel owner Disney wasn't keen on the idea. Black says:
"I think we were just told by the studio that we should probably paint Tony Stark as being kind of an industrialist and a crazy guy, or even a bad guy at some points, but the 'Demon in a Bottle' stuff of him being an alcoholic wouldn't really fly.
I don't blame that. I wouldn't be surprised if at some point someone wanted to make a movie and they'd run out of directions for the character, then they've still got 'Demon in a Bottle'"
Co-writer Drew Pearce added that alcoholism probably wouldn't have made for a compelling villain in a big-budget Hollywood superhero movie.
Source: Comic Book Movie