If you believe Bob Woodard’s bio of John Belushi, Wired, the film version of The Blues Brothers was held up for weeks mainly due to the fact that Belushi & Carrie Fisher spent so much time practically barricaded in Belushi’s trailer getting stoned.
Also, the 1963 “epic” Cleopatra is the quintessential case study of a film ruined by disastrous backstage problems. Not the least of which was that stars Elizabeth Taylor and Richard Burton were clearly more interested in having an affair and indulging in massive appetites for food, alcohol and drugs.
Another example of a movie ‘ruined’ by drug use (though not in the way the OP had in mind) was the mid 90s indie film The Watermelon Woman. This film is a fictional account of the RL film-maker trying to make a documentary about a 30s actress known only as “the Watermelon Woman.” It’s an interesting conceit, and the film touches on the topics of the depiction of race both in race and in real life.
At one point in the film, police officers see the young (African-American) film-maker walking along the street with expensive film equipment and do a stop & search on her. They demand to know where she got the cameras, frisk her, and examine her for signs of drug use. The implication being that a black woman carrying expensive looking MUST HAVE stolen it, with the intent of selling it for drug money.
Now, this is a very real problem in America, and one deserving of being explored in a movie. There’s just one problem…the central character IS shown in several scenes clearly smoking pot. No, it’s not crack but it pot was still an illegal drug in '96. And people have stolen things to fuel even a pot habit. I kept watching the movie wondering “How am I supposed to empathize with a person lamenting that she’s being unfairly labelled a drug user, when in fact, she IS a drug user??” It was a rather awkward flaw in a movie I otherwise liked a lot.