Using the internet for excessively long periods can give people withdrawal symptoms similar to drug abusers, scientists have warned.
Researchers found that people were left in 'negative moods' after clocking up long hours surfing the net.
Their feelings were similar to people coming down after using recreational drugs like ecstasy.
Just like drug users, when people went back on the internet their negative moods lifted.
The research was carried out on 60 volunteers with an average age of 25 at Swansea University's College of Human and Health Sciences.
Scientists say the results could mean society is in for some 'nasty surprises' if internet use increases as expected.
Professor Phil Reed, of Swansea University's Psychology Department, said: 'Our results show that around half of the young people we studied spend so much time on the net that it has negative consequences for the rest of their lives.
'When people come off-line, they suffer increased negative mood - just like people coming off illegal drugs like ecstasy.
'These initial results, and related studies of brain function, suggest that there are some nasty surprises lurking on the net for people's wellbeing.'
The study, published in the international journal Plus One, is the first of its kind into the immediate negative psychological impacts of internet use.
A Swansea University spokesman said: 'A total of 60 volunteers with an average of 25 were given a series of psychological tests to explore levels on internet addiction , mood anxiety and depression.
'They were then given exposure to the internet for 15 minutes and re-tested for mood and levels of anxiety.
'The negative impact of excessive internet use can be seen across a wide range of aspects of the addict's life.'
The results of the study also showed that heavy web users also tend to be more depressed and show higher levels of autism traits.
