I've answered this already here: https://www.reddit.com/r/AskHistorians/comments/34cof1/how_are_cigarettes_legal_in_the_usa_how_did/cqtt6mz?context=3

Rather than reply to a four month old thread (linked below), I'll just add this: Alcohol was banned. In fact, alcohol was banned before Marijuana. The 18th Amendment and the Volstead Act predate the Marijuana Stamp Act by near twenty years. It's just that it was tried and failed before an attempt to ban marijuana. The ban on marijuana has just been far more successful.