“Jennifer’s Body” is not scary enough for horror and not knowing enough for commentary on horror.

“Hell is a teenage girl,” says Needy (Amanda Seyfried of “Mama Mia” and “Big Love”). And she should know. She is a teenage girl. And her Best Friend Forever Jennifer (made-for-Maxim hottie Megan Fox of the “Transformers” films) is both, quite literally. She may wear a BFF charm in recognition of friendship going back to the sandbox days, but she is more of a frenemy, one of those people who are in our life and have great powers of influence even though we are not sure why and we don’t like them very much. Jennifer tells the appropriately-nicknamed Needy (for Anita) to break a date with her boyfriend Chip (Johnny Simmons) and accompany Jennifer to the town’s only club/bar so they can watch an up-and-coming group called “Low Shoulder.” Needy says yes to Jennifer, no to Chip, and they are on their way.

Some nasty things happen, and Jennifer turns into a demon who preys on the bodies of helpless and terrified victims. (They have to be helpless and terrified; she’s something of a picky eater in that respect.) Needy is the only one who sees her for what she is, and as the remains begin to pile up and funerals clog everyone’s schedules, Needy has to figure out what to do. And it wouldn’t be a horror movie if it didn’t get personal.

When Diablo Cody (“Juno” and many opinionated but very cool and clever essays in Entertainment Weekly) takes on the horror genre, it is fair to expect some snappy honest-to-blog patter and if not obvious po-mo air quotes at least something a little meta. But it does not work as straight-on horror or as what has now become an established and rich little sub-genre of the snarky, self-aware horror film that delivers on two levels, like the “Scream” series, “Buffy the Vampire Slayer,” or “Shaun of the Dead.”

It actually is not that much of a stretch between “straight” horror and meta-horror. Straight-on horror films deliver thrills with a combination of the predictable (woe to any teenager who has sex and if the camera zooms in close, there’s something terrible just out of frame) and the unpredictable (you know, the part where some one or some thing jumps out and splits some hapless victim down the middle). And they often have a sense of humor about themselves and their conventions, with serial killers Freddy Kruger (of “Nightmare on Elm Street”) and Chuckie (the demon doll) tossing off wisecracks and bad puns as they slaughter.

Cody has a few nice points to make here about the frenemy dynamic of some girl-girl relationships: the drama, the connection, the power plays, and the excitement. “Not high school evil; evil-evil” is an theme worth exploring, but it is hard to improve on “Buffy.” The idea of the indie band’s song “Through the Trees” being used at the candlelight vigils for the dead classmates is well-handled. And there are some cutesy winks at the audience, as when the indie band talks about getting on a movie soundtrack as a path to fame and fortune, as happened to the Moldy Peaches on the soundtrack of “Juno.” Casting everyone’s favorite boy next door Adam Brody as an “agent of Satan with a really awesome haircut” is fun. Director Karyn Kusama and cinematographer M. David Mullen find some striking compositions, though it is unfortunate that a pool scene recalls the much better teen vampire movie “Let the Right One In.” And what is the deal with that 80’s prom dress? Now that’s horror.