High Times Future

Science Fiction-- particularly of the "New Wave" of The Sixties and The Seventies-- assumed that increasing liberalization of drugs would continue and that drugs currently banned would become not only legal but also as popular and commonplace as alcohol and tobacco are (or were, now that tobacco is becoming more stigmatized). Usually the drugs in question are "soft" drugs like marijuana and the hallucinogens (particularly LSD) rather than cocaine or heroin; often Fantastic Drugs are part of the mix as well. Some works decide to be ironic and legalize currently illegal drugs, but have alcohol and tobacco banned.

Unlike Government Drug Enforcement, there is no compulsion to take such drugs, and May Contain Evil doesn't necessarily come up (unless a new drug is luring people away from the popular stuff).

This is somewhere between a Discredited Trope and a Forgotten Trope these days, what with Drugs Are Bad having been enforced in recent years. Could come back as marijuana legalization gets more discussion, or remain the occasional Author Appeal topic.

Contrast with Eternal Prohibition.

Comics

 * Transmetropolitan, which leans heavily on the science fiction of this era for inspiration and setting, runs on this trope.

Literature

 * Bug Jack Barron: Jack's talk show is sponsored primarily by Acapulco Golds, "America's Premium Marijuana Cigarettes" (the book was written when cigarette ads were still legal on U.S. television). Tobacco is illegal.
 * A Clockwork Orange: Mescaline (or a synthesized form) is apparently legal since you can get it added to your yummy glass of moloko.
 * The Eighth Doctor Adventures novel Alien Bodies featured "Cloud Nine: The Original Cannabis Cigarette". As smoked by UNISYC troopers.

Live Action TV

 * Dragnet characters-of-the-week seemed to love this trope. It showed up repeatedly, usually to be met with angry rebuttals by Sgt.Friday.

Western Animation

 * Alluded to in Futurama with a brief gag about a vending machine that sells "Refreshing Crack".