Can you log off long enough to see if you have brain damage?
They're often gamers who go without food or drink, ignoring their education, work and relationships.
Internet addiction, in which individuals spend an excessive amount of time online, is recognised by some experts as a medical condition.
But for the first time Chinese scientists say they have found that the problem can be as damaging as being hooked on heroin or cocaine.
Sufferers report a compulsion to stay online for longer and longer periods, then experience withdrawal symptoms such as depression when trying to cut down.
The groundbreaking study of 17 adolescents from a mental health centre in Shanghai showed that the neuron fibres in the white matter in their brains were less healthy than normal. Similar damage has been found in alcoholics and drug addicts.
Researchers hope the findings will lead to the development of new ways to treat a problem that is taken very seriously on the mainland. Last year a survey by the China Youth Internet Association found more than 24 million young people in cities were internet addicts, and 18 million showed early symptoms.
With internet use spreading rapidly, the questions below provide a quick tell-tale sign of trouble.
If you answer 'yes' to the first five questions below and at least one of the remaining three questions then you are an internet addict.
1. Do you feel preoccupied with the internet (i.e. think about previous online activity or anticipate your next online session)?
2. Do you feel the need to use the internet for increasing lengths of time in order to achieve satisfaction?
3. Have you repeatedly made unsuccessful efforts to control, cut back or stop using the internet?
4. Do you feel restless, moody, depressed or irritable when attempting to cut down or stop using the internet?
5. Do you stay online longer than originally intended?
6. Have you jeopardised or risked the loss of a significant relationship, job, educational or career opportunity because of the internet?
7. Have you lied to family members, a therapist or others to conceal the extent of your involvement with the internet?
8. Do you use the internet as a way of escaping from problems or of relieving a distressed mood (e.g. feelings of helplessness, guilt, anxiety and depression)?