Tell your children: Reefer Madness is not for them. But those of legal age — and perhaps illegal interests — will get a kick out of the charcoal-black comedy, a winking update of 1936’s celluloid sermon and the reincarnation of a frill-free stage show. The supersize budget is put to good use in a swirl of gore-happy special effects and elaborate song-and-dance sequences. And the cast sure can carry a tune. Most noteworthy: sweet Mary Lane (Veronica Mars star Bell), lashing out in kitten-with-a-whip mode. S&M has never had such a catchy beat. EXTRAS ”It’s like Fiddler on the Roof for the crack generation!” jokes screenwriter-composer-executive producer Dan Studney in ”Grass Roots: Behind the Scenes.” Studney and his cast and crew keep things quippy in a commentary track, whether they’re discussing instrumentation (using electric guitar for racy drug ditties versus acoustic guitar for G-rated ballads) or influences (Busby Berkeley, revered by ’60s and ’70s potheads). But the biggest howler is the inept black-and-white original, included here, which hasn’t gotten any better over time.