In trying to think of some way to combine the arbitrary pothead holiday 4/20 with my regularly scheduled musings about sci-fi movies, I slowly realized something...Sci-Fi may just be the most weed-friendly genre. Think about it.
In horror movies, the punishment for taking even a few puffs is always having a machete rammed violently through the offender's lungs (Friday the 13th, Nightmare on Elm Street, The Tripper). In dramas, Mary Jane is either a gateway drug to a disproportionately harsh life of hard drugs and even harder living (Requiem for a Dream, Haven), or it's a sign that someone's perfect life isn't quite so pristine (American Beauty). Even in comedies pot is often the juvenile hurdle one must get past to become an adult (Knocked Up, The Wackness). Plus, it's always getting stoners into trouble — funny, shennanigan-ensuing trouble, but trouble nonetheless.
Pot in sci-fi, though, is usually harmless, and often kind of a good thing. Think about its presence in Children of Men, Altered States, The Cell, Eternal Sunshine of the Spotless Mind, Invasion of the Body Snatchers and Battlestar Galactica. Brilliant scientists and saviors of the human race can be seen smoking up no differently than in other genres and rarely do they get their cinematic comeuppance for doing so. They're not killed for using it. They never run into trouble with drug dealers. Their life doesn't spiral out of control because of it. And, perhaps best of all, they're never judged by their peers for smoking it. Pot in sci-fi is just there, like in real life, to be enjoyed in an entirely recreational fashion.
What's even more interesting to me is that sci-fi doesn't give a similar pass to all drugs, just marijuana. Granted this probably has more to do with writers wanting to speculate on drug usage in alternate realities or the future than it does some latent respect for grass, but sci-fi stories are far more concerned with more dangerous designer drugs than simple, from-the-earth pot. Even still, by always painting a target on another drug's back, sci-fi is acquiescing to the idea that pot really is in a small-time league of its own as far as (currently) illegal substances go.
Having said all that, there's actually not a whole lot of pot usage in sci-fi movies. Then again, if one wants to get statistical about it, there are simply far fewer sci-fi films than most genres (so few, in fact, that sci-fi has become an umbrella for movies that have no respect for actual science fiction); so in my book that's still not evidence against sci-fi being the most weed-friendly genre. Also, considering there is a dearth of usage but no great lack of references to the drug, I'd argue that has more to do with most sci-fi movies these days being rated PG-13 than anything else.
And let us not forget that in addition to being largely judgment-free of its pot-loving characters, sci-fi is even friendlier to its pot-loving audience members. No one ever says, "But have you seen The House of Sand and Fog...ON WEED?!" but if a movie has far-out, mind-bending ideas or mind-blowing special effects, you can bet your medical marijuana license that at some point one pothead has recommended to another that they watch it high for a whole different kind of experience. Sure, that's not exclusive to sci-fi — don't potheads want to watch everything high? — but if one wanted to be anal about it and set up a polling station on a college campus, I'd say it's a safe bet most people who are holding out for watching some variation of sci-fi high more often than any other genre.