Stephen King's 'On Writing - A Memoir of the Craft' is a most interesting read. Stephen talks about his early days and his struggle with drugs, especially alcohol. Two other famous twentieth century writers who were substance-abusers include Hemingway and Fitzgerald (writer of the month). King writes of his justification for drinking in his book.
'I also employed the world famous Hemingway Defense. Although never clearly articulated ( it would not be manly to do so), the Hemingway Defense goes something like this: as a writer, I am a very sensitive fellow, but I am also a man, and real men don't give in to their sensitivities. Only sissy men do that. Therefore I drink. How else can I face the existential horror of it all and continue to work?'.
Fitzgerald was an alcoholic since his college days and his drinking became heavy as the years went by. Alcohol left him in poor health and subsequently resulted in his death.