Web addicts have brain changes similar to those hooked on drugs or alcohol, preliminary research suggests.
Experts in China scanned the brains of 17 young web addicts and found disruption in the way their brains were wired up.
They say the discovery, published in Plos One, could lead to new treatments for addictive behaviour.
Internet addiction is a clinical disorder marked by out-of-control internet use.
A research team led by Hao Lei of the Chinese Academy of Sciences in Wuhan carried out brain scans of 35 men and women aged between 14 and 21.
Seventeen of them were classed as having internet addiction disorder (IAD) on the basis of answering yes to questions such as, "Have you repeatedly made unsuccessful efforts to control, cut back or stop Internet use?"
Specialised MRI brain scans showed changes in the white matter of the brain - the part that contains nerve fibres - in those classed as being web addicts, compared with non-addicts.
There was evidence of disruption to connections in nerve fibres linking brain areas involved in emotions, decision making, and self-control.
Dr Hao Lei and colleagues write in Plos One: "Overall, our findings indicate that IAD has abnormal white matter integrity in brain regions involving emotional generation and processing, executive attention, decision making and cognitive control.
"The results also suggest that IAD may share psychological and neural mechanisms with other types of substance addiction and impulse control disorders."
Prof Gunter Schumann, chair in biological psychiatry at the Institute of Psychiatry at King's College, London, said similar findings have been found in video game addicts.
He told the BBC: "For the first time two studies show changes in the neuronal connections between brain areas as well as changes in brain function in people who are frequently using the internet or video games."
Commenting on the Chinese study, Dr Henrietta Bowden-Jones, consultant psychiatrist and honorary senior lecturer at Imperial College London, said the research was "groundbreaking".
She added: "We are finally being told what clinicians suspected for some time now, that white matter abnormalities in the orbito-frontal cortex and other truly significant brain areas are present not only in addictions where substances are involved but also in behavioural ones such as internet addiction."
She said further studies with larger numbers of subjects were needed to confirm the findings.