It’s hard to miss those jarring black and white advertisements around the city that say, in scrawled block letters, “My Mom Always Hated You, Sarah Marshall,” and “You Do Look Fat in Those Jeans, Sarah Marshall,” and “You Suck Sarah Marshall.” You’re left wondering, Who is that poor girl? And why is there a nasty smear campaign against her?

Those who aren’t too turned off to ascertain what these aggressive ads are all about have discovered that they’re ads for a new Judd Apatow-produced movie, “Forgetting Sarah Marshall,” which debuts on April 18. The 40-year-old Mr. Apatow wrote and directed “The 40-Year-Old Virgin” and more recently, “Knocked Up.”

But no one would get from the ads that the movie itself happens to be about a 26-year-old man, Peter, who’s just been dumped by his unfaithful girlfriend, Sarah Marshall.

The film is not another “Mean Girls” or “Heathers,” which parodied teenage cruelty in an effort to expose it.

But it is the ads that are our subject today. They are supposed to depict Peter’s revenge, a public smear campaign of his ex. But they sound like catty adolescent girls talking about an ostracized classmate — not an infrequent occurrence in reality.

If you are confused, you are supposed to be. The “Sarah Marshall” ads are a new chapter in the viral marketing trend, which generates curiosity through vague messages. The recent film “Cloverfield” rode its own clever viral marketing campaign — which included ambiguous TV commercials, cheeky product placement, and artificially-generated fan sites — to a huge opening weekend at the box office.

But the “Sarah Marshall” ads eschew that clever spirit for blunt, easy impudence. These ads are toxic — not the direction viral marketing should go.

The president of marketing and distribution for Universal Pictures, Adam Fogelson, said the goal of the ads was simply to get the name “Sarah Marshall” to stick.

It’s not the name, though, that many people are remembering, but rather the notion that someone was bitter enough to replicate their anger on public billboards around New York and other cities. The signs seem to endorse what’s the worst of girls — or men, as it turns out.

Many New Yorkers, in fact, have no way of knowing that these are movie advertisements, and are left simply with an unpleasant sentiment.

“My first impression was outrage,” a teacher at the Grace Church School in Manhattan and a mother of two young children, Ilana Laurence, said. “There was no indication that it was linked to a film, or I couldn’t see one. Initially, I thought some girl is doing this to poor Sarah Marshall. These ads are outrageous and the campaign is stupid.”

“My reaction was, ‘Ugh, you’ve got to be kidding me,'” an assistant professor of child and adolescent psychiatry at the NYU Child Study Center, Jean-Marie Bruzzese, said. “The ads have nothing to do with the brand and are spreading pathological messages to teenagers. I think it’s dangerous for them to see those messages without an understanding of the big picture. And the fact that they are shown in places they frequent is alarming.”

Mr. Apatow is known for a particular brand of comedy that targets men between the ages of 16 and 40. His films obviously hit the right spots with their core demographic, but they don’t always depict women in the best light. Mr. Apatow did not make the ads but he was consulted on them. The ads, though, perpetuate a vicious cycle of nastiness, particularly among adolescent girls.

“I think because the ads are so in-your-face and prevalent, it’s giving a message to students that this is an okay way to react to break-ups, and it gives them permission to do something similar — to copy the behavior,” Ms. Bruzzese said.

Gossip Web sites, such as JuicyCampus.com, already feature malicious postings that are similar to the tone of the “Sarah Marshall” ads.

Cyberbullying, which is the essence of Peter’s fictional Web log, ihatesarahmarshall.com, is a serious problem among schoolchildren.

What might be most disturbing is that the ads blur the line between reality and entertainment. In particular, Peter’s faux blog on the film’s Web site looks like a real one that teenagers populate.

“It seems very intentional that you really feel like you are visiting a Web site about a real person,” the author of “Odd Girl Out: The Hidden Culture of Aggression in Girls,” Rachel Simmons, said. “It’s hard for young kids to distinguish between something that’s entertainment and what’s real.”

Moreover, those ads are a bit off — not just in a branding sense, but also psychologically — because what they’re really about is harassing someone. “The ads make that behavior seem glamorous and cool and that’s really crossing the line,” Ms. Simmons said.

“We understand it’s edgy, but don’t think it’s beyond the boundaries,” Mr. Fogelson countered. “Looking fat in jeans and looking fat are not the same thing. Saying you look fat in those jeans is totally acceptable. It’s a cute comment that exists between partners.”

Really? Someday viewers will find the guts to turn away when they see these kinds of comments — whether in an ad or by someone real. Maybe they already are.

But there is a final problem. The very topic of the ads is going to be subject for talk. It’s too bad because it all distracts from what turns out to be a surprisingly good film.

kherrup@nysun.com