For years, most moviegoers didn’t know the difference between green-band trailers (featuring content approved by the Motion Picture Assn. of America for “appropriate audiences”) and their “restricted” red-band counterparts (dubbed for the red slate that appears at the head of the trailer). At one point, studios nearly abandoned red-bands entirely, believing it wasn’t cost-effective to produce advertising that could only run in front of select R and NC-17 features.

Today, thanks to the Internet, everyone seems to be jumping on the red-band wagon. From “This Is the End” to “Piranha 3DD,” previews packed with violence, profanity, nudity and drug use instantly go viral after debuting online, encouraging marketing departments to craft edgier spots for the Web, where they can target extremely focused demographics. To appease the MPAA, sites “age-gate” the content — an ineffective honor system that requires users to enter their birthdates before viewing.

“Red-bands have been around online for a while, but it’s only in the last year or two that there’s really a mechanism to effectively disseminate those materials,” says Radius marketing and distribution VP Heath Shapiro, who attributes the success of last year’s “Bachelorette” to a red-band trailer launched on FunnyOrDie.com. “There are just all these portals clamoring for content like that, which in tandem with social media can really amplify something in a moment’s notice.”

The rise of the red-band started with laffers. As comedies got raunchier in the hands of Judd Apatow and others, the only truly representative way to market them was to reveal some of the dirty jokes they had in store. Universal went all out with Ted last summer, fully embracing the film’s irreverent sense of humor by creating reams of red-band trailers, clips and even an MPAA-restricted tie-in campaign with Axe men’s products.

In the past couple months, studios have extended the red-band trend to a range of genre movies, believing the most effective way to win over the adult fans of extreme horror (“Evil Dead”), intense thrillers (“Trance”) and Michael Bay mayhem (“Pain & Gain”) is to give them a taste of the R-rated content.

Earlier in April, Radius debuted a punchy red-band teaser for Nicolas Winding Refn’s Only God Forgives on Yahoo Movies. Word of the trailer drop spread quickly as other entertainment sites picked it up as breaking news. Per a Radius rep, the teaser was the top subject trending on Twitter within the hour.

Sony had a similar success with promos for “Evil Dead.” The campaign led with a gory red-band teaser online that featured scenes of everything from bloody power tools to a seductive young lady splitting her tongue on a box cutter. Once die-hard fans had “discovered” the teaser, the studio followed up with a tamer green-band to broaden their audience.

“I think what has worked with comedies holds true with Evil Dead and the horror genre,” says TriStar and Screen Gems marketing head Loren Schwartz, whose resume also includes red-band trailers for “Superbad” and “Pineapple Express.” “With “Evil Dead,” we have a very hard-R picture that has a huge fanbase who wouldn’t accept anything less. Selling only a diluted version of that movie would not be fair.”

The red-band phenom has gotten so popular that even media that doesn’t answer to the MPAA have tried to co-opt the strategy. Earlier this month, Web promos for Netflix’s “Hemlock Grove” series posed as restricted trailers, opening with a red advisory screen.

The strategy has its limits, however. By including extreme content, studios run the risk of giving people reasons not to go. That’s not necessarily a bad thing, according to Shapiro. For films with more than just opening-weekend receipts in mind, “You want to embrace an audience that’s going to embrace the movie,” he says.

Fox Searchlight reps felt the same way about “Trance”: “We are really cognizant of establishing the correct tone and atmosphere of these kinds of movies, but you can only show so much in your green-band trailer,” explains Searchlight marketing honcho Larry Baldauf, who followed up a U.K.-made all-audiences trailer for “Trance” with a special Web-only red-band. TV spots running during Adult Swim and other latenight programming blocks slyly suggested that viewers also seek out the red-band trailer on Shazam.

“One of the selling points we wanted to focus on was that this was Danny Boyle returning to “Shallow Grave” form, that this was an experiential film, and this was something more than a standard heist caper movie,” Baldauf says. So the team started looking for what he calls “sticky imagery” — moments that audiences wouldn’t forget.

Lucky for them, “Trance” is rife with striking visuals. The marketing team settled on a startling, hallucinatory moment in which Vincent Cassel’s character gets the upper half of his head blown away … and then picks himself off the floor and keeps right on talking.

“That shot — and we make you wait a minute and a half for it — is what justified the red band,” Baldauf says. “In a movie with a traditional narrative, that could be a spoiler, but here it’s just part of the fever dream. Anytime you have signature imagery like that, something you’ve never seen before, it’s really sticky.”