The classic from 1936. From Wikipedia:
Reefer Madness (aka Tell Your Children) is a 1936 American exploitation film revolving around the tragic events that ensue when high school students are lured by pushers to try "marihuana": a hit and run accident, manslaughter, suicide, rape, and descent into madness all ensue. The film was directed by Louis Gasnier and starred a cast composed of mostly unknown bit actors. It was originally financed by a church group and made under the title Tell Your Children.
The film was intended to be shown to parents as a morality tale attempting to teach them about the dangers of cannabis use. However, soon after the film was shot, it was purchased by producer Dwain Esper, who re-cut the film for distribution on the exploitation film circuit. The film did not gain an audience until it was rediscovered in the 1970s and gained new life as a piece of unintentional comedy among cannabis smokers. Today, it is in the public domain in the United States and is considered a cult film. It inspired a musical satire, which premiered off-Broadway in 2001, and a Showtime film, Reefer Madness, based on the musical.