Internet addiction disrupts nerve wiring in the brains of teenagers, a "groundbreaking" study has found.
Similar effects have been seen in the brains of people exposed to alcohol, cocaine and cannabis.
The discovery shows that being hooked on a behaviour can be just as physically damaging as addiction to drugs, scientists believe.
Internet addiction disorder (IAD) is a recently recognised condition characterised by out-of-control internet use.
Sufferers spend unhealthy amounts of time "online" to the extent that it impairs their quality of life.
Denied access to their computers, they may experience distress and withdrawal symptoms including tremors, obsessive thoughts, and involuntary typing movements of the fingers.
Until now research on IAD has focused on psychological assessments.
The new study, from China, used a Magnetic Resonance Imaging (MRI) technique to look at its effects on brain structure.
Scans were carried out on 17 internet-addicted adolescents and 16 non-addicted individuals, and the results were compared.
In the IAD-diagnosed teenagers, the scientists found evidence of disruption to "white matter" nerve fibres connecting vital parts of the brain involved in emotions, decision making, and self-control.
A measurement of water diffusion called "fractional anisotropy" (FA) was used which provides a picture of the state of nerve fibres. Low FA was an indicator of poor nerve fibre structure.
The researchers, led by Dr Hao Lei from the Chinese Academy of Sciences in Wuhan, wrote in the online journal Public Library of Science ONE: "Our findings suggest that IAD demonstrated widespread reductions of FA in major white matter pathways and such abnormal white matter structure may be linked to some behavioural impairments.
"In addition, white matter integrity may serve as a potential new treatment target."
Previous studies had shown abnormal white matter structure in the orbito-frontal regions of the brains of people exposed to alcohol, cocaine, cannabis, methamphetamine and ketamine, said the researchers.
They added: "Our finding that IAD is associated with impaired white matter integrity in the orbito-frontal regions is consistent with these previous results."
The scientists suspect the damage is caused by disrupted myelin, the fatty insulating sheath that coats nerve fibres and helps them to function.
Commenting on the findings, Dr Henrietta Bowden-Jones, consultant psychiatrist at Imperial College London, said: "This type of research exploring the differences between normal brains and brains of people who suffer from internet addictions is groundbreaking, as it makes clear neuroimaging links between internet addiction and other addictions such as alcohol, cocaine and cannabis amongst others.
"We are finally been 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."
Currently, internet addiction is officially classified as an "impulse control disorder" rather than a genuine addiction.
Further studies with larger numbers of subjects would be needed before consideration could be given to reclassifying it, said Dr Bowden-Jones.
She added that it is "possible to consider this study as one of the seminal papers in determining how future generations of clinicians will view internet addiction".
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