Every English/writing teacher who's spoken to me on the subject has insisted that alcohol and drugs do not improve their writing. Every friend, peer, and person who is not being paid to influence young minds who has spoken to me on the subject, as well as my own experience, says that they do help.

I've personally only written while drunk. My girlfriend says that nicotine helps her. I've heard that marijuana helps, up to a point; after that point, the writing tends to devolve into "omg i'm so high right now" rather than anything of substance. I don't have many accounts of other drugs, although the frequency with which Adderall is used in colleges seems to suggest it's useful for academia.