Eating disorder support groups are warning that a growing number of websites are pushing vulnerable people into anorexia and bulimia.
An estimated 1m people in the UK have an eating disorder
Experts say so-called "pro-ana" sites claim to offer support, but actually glorify the diseases and discourage people from seeking medical treatment.
They say these sites give tips on how to lose weight and keep it off, and present the illnesses as normal.
One doctor is urging service providers to shut the sites when they appear.
A young university student, who uses the online identity "Kasia", explained the appeal of the websites.
"I think I am quite glad that I found them because otherwise I would have just felt absolutely, totally alone," she told BBC Radio Five Live.
"You don't choose to be like that. It can just be that you feel that your life is so empty that all you can do is try and change yourself to kind of put meaning there," she said.
But Dr Dee Dawson, Medical Director at Rhodes Farm, who treats people with eating disorders says the sites promote the diseases as being "normal".
"Really they're not in recovery these people. They're really in the grips of a life-threatening illness and they're trying very hard to promote that illness as being normal and wanting to recruit people into their very weird club.
"I think the servers should shut them down immediately they see them on the computers," she told BBC Radio Five Live.
Amanda Hills, a psychologist and counsellor at the National Centre for Eating Disorders said the sites "keep people in their illness" instead of helping them out of it.
"The danger of them is that actually it's for people who actively choose to be ill and it perpetuates the illness in a way that it encourages them not to seek treatment.
"Seeking treatment is the way forward but these sites actually seek to keep people in their illness by giving them tips and all sorts of things that they might not have thought of themselves."
Steve Bloomfield from the Eating Disorders Association said the sites could appear "seductive" to those with anorexia or bulimia.
"They extol it as a lifestyle , as a choice and because of that, when people come onto these sites ... they find somebody who has an eating disorder who really seems to understand what it's like."
The mental health charity Sane carried out a survey of people using pro-ana sites.
Preliminary analysis showed 97% respondents were women between 13 and 49 years, with 41 % aged between 16 to 19.
Almost half the respondents visited a site several times a day.
Marjorie Wallace, chief executive of Sane, said: "Sane's concern is that these sites encourage people to hold onto their illness, reinforcing their belief and supporting them in a pattern of damaging behaviour, without encouraging them to seek treatment.
"We believe there are serious dangers in people relying on theses websites, using them as a substitute for social and family contact and emotional support.
"We call on those publishing these sites to review their content urgently and to remove extreme exhortations and images and take responsibility for messages which sustain self-destructive and potentially lethal behaviours."