ANONYMOUS trolls are convincing young people to deliberately harm themselves in pop star Justin Bieber's name.

A hashtag is trending worldwide after a website declared: "Let's start a cut yourself for Bieber campaign… see if we can get some little girls to cut themselves".

Now many people are posting self-harm pictures on Twitter, although it's not clear which are real. Miley Cyrus quickly chimed in, retweeting:

"Cutting is NOT something to joke about. There are people who are actually suffering from self-harm, this is so disrespectful."

Self harm, like cutting, is when people deliberately hurt themselves without necessarily wanting to commit suicide.

The sick joke began after allegations emerged that Bieber was smoking drugs.

Fans then appeared to take it seriously, posting disturbing messages such as "you stop using drugs and we'll stop cutting" along with pictures of bleeding arms.

Others took it as a joke, posting pictures of themselves cutting food, cutting the lawn, or splashing themselves with fake blood.

BeyondBlue warn that self harm is a serious issue and is often a way for people to deal with distress, anxiety or depression.

Dr Brian Graetz, general manager of child and youth, said cutting could be a significant call for help, not a joke, and that social media had a strong peer influence.

"They're introducing a whole new group of kids to cutting… I don't think kids need to go there," he said.

"This is a real concern."

Dr Graetz said young people often couldn't express themselves with words so they sometimes turned to dysfunctional behaviour.

But hurting yourself to protest someone else's perceived harmful behaviour was dangerous, he said.

"I don't think we should look at this as being something we can have a laugh at, it does have some serious consequences," he said.

BeyondBlue say anyone who has self harmed should seek immediate treatment for their injuries and longer term treatment for their issues.

For further information or to get help, visit BeyondBlue or call them on 1300 22 4636, or try Lifeline or Sane.