There is a strong link between heavy internet use and depression, UK psychologists have said.The study, reported in the journal Psychopathology, found 1.2% of people surveyed were "internet addicts", and many of these were depressed.
The Leeds University team stressed they could not say one necessarily caused the other, and that most internet users did not suffer mental health problems.
The conclusions were based on 1,319 responses to an on-line questionnaire.
Recruitment was via links on social networking sites. People were asked how much they used the internet and for what purposes.
They were also asked a series of questions to assess whether they suffered from depression.
If a web addict is substituting meaningful friendships and socialising with virtual contact on the internet, this might have an adverse affect on their mental wellbeing
Sophie Corlett of the charity Mind
The respondents were aged 16 to 51, with an average age of 21.
The authors found that a small number of users had developed a compulsive internet habit, replacing real life social interaction with online chat rooms and social networking sites.
They classed 18 respondents - 1.2% of the total - as "internet addicts".
This group spent proportionately more time on sex, gambling and online community websites.
'Darker side'
Lead author Dr Catriona Morrison said: "The internet now plays a huge part in modern life, but its benefits are accompanied by a darker side. "While many of us use the internet to pay bills, shop and send e-mails, there is a small subset of the population who find it hard to control how much time they spend online, to the point where it interferes with their daily activities." The internet addicts were significantly more depressed than the non-addicted group, with a depression score five times higher. The average score of the internet-addicted group put them in the category of moderate-to-severe levels of depression.
There is no good evidence that the problem is the internet itself
Dr Vaughan Bell of King's College London
"Our research indicates that excessive internet use is associated with depression, but what we don't know is which comes first - are depressed people drawn to the internet or does the internet cause depression?" said Dr Morrison.
"Now we need to investigate the nature of that relationship and consider the issue of causation."
'Emotional distress'
Critics of the research say that internet addiction cannot be diagnosed reliably.
Dr Vaughan Bell, from the Institute of Psychiatry at King's College London said that by definition, those identified as "internet addicts" are emotionally distressed, so the conclusions are "not a big surprise".
In terms of cause and effect, he pointed out that previous research has suggested that people who are depressed or anxious may be more likely to use the internet rather than the other way round.
He added: "There are genuinely people who are depressed or anxious who use the internet to the exclusion of the rest of their lives, but there are similar people who watch too much TV, bury themselves in books or go shopping to excess. "There is no good evidence that the problem is the internet itself." Mental Health charities said the way people spend their time and the kind of social interaction they engage in could well impact on mental wellbeing.
"Social connections"
Dr Andrew McCulloch, chief executive of the Mental Health Foundation, pointed out that, in some ways, the internet can be helpful. He said: "To the extent that the internet encourages meaningful friendships and social connections it can be a very good influence on people's lives. "However, social interaction online should not usually replace an offline social life. We should take note of this study's findings - it suggests that further research in the area is needed." Sophie Corlett, of the mental health charity Mind, said: "Evidence suggests that active pursuits such as exercise and socialising with people face-to-face are among the factors that help us stay in good mental health. "Although excessive internet use can't be said to cause mental health problems, if a web addict is substituting meaningful friendships and socialising with virtual contact on the internet, this might have an adverse affect on their mental wellbeing."