Young adults who were bullied during childhood are more than twice as likely to suffer from depression, research has suggested.
A study, which analysed more than 6,700 young people in the UK, concluded that almost a third (29.2 per cent) of depression at the age of 18 could be explained by peer victimisation.
The team of scientists, led by research fellow Lucy Bowes at the University of Oxford, found that just over five per cent of teenagers who did not experience bullying had depression, increasing to nearly 15 per cent in those who suffered it frequently.
They said anti-bullying programmes in schools could be an effective way of reducing depression in adulthood, although they described previous ones based on cognitive behavioural therapy as "disappointing".
The study found that of the 683 participants who reported frequent bullying at the age of 13 years, 101 (14.8 per cent) suffered from depression at the age of 18, while of the 1,446 who said they experienced some bullying, 103 (7.1 per cent) were depressed at 18.
Of the 1,769 participants who reported no victimisation at 13, 98 (5.5 per cent) were depressed by the time they reached 18.
The most commonly-reported type of bullying was being called nasty names or having personal belongings taken.
Most teenagers (41-74 per cent) reported that they never told their teachers while 24-51 per cent said they never told their parents.
Teenagers were most likely to report physical encounters such as being hit or beaten up
Researchers said that compared with those who were not bullied, adolescents who were bullied at 13 were more likely to be female and more likely to have displayed higher levels of emotional and behavioural problems before being bullied.
"Our findings suggest that approximately 29 per cent of the burden of depression at age 18 years could be attributed to peer victimisation," the study authors concluded.
"These findings lead us to conclude that peer victimisation during adolescence may contribute significantly to the overall public health burden of clinical depression and that intervention to reduce peer victimisation in secondary schools should reduce the burden of depression."
Last month a study published in the journal Psychological Medicine found that being bullied at school has effects on the body that last for decades and can shorten a person's life.
Scientists found a significantly-increased risk of stress-induced chronic inflammation in middle-aged men and women who had been bullied as children, increasing the chances of having blocked arteries and leading to a risk of fatal heart attacks and strokes.
In women, falling victim to childhood bullying was also found to raise the risk of being clinically obese in later life by about 40 per cent, the study, which was co-led by the Institute of Psychiatry, Psychology and Neuroscience at King's College, London, said.
An NSPCC spokesman said: "Online bullying is a serious and growing problem, with calls to ChildLine about it alone almost doubling last year. Young people tell us how desperately unhappy they are because of its 24/7 nature, and the feeling that they can't escape this relentless harassment.
"Bullying can have a devastating impact on a child. It can affect their feelings of self-worth, and potentially trigger depression.
Last year, bullying was the second biggest worry for young people aged 12-15 years old contacting ChildLine, and one in four under-11s using the service were counselled about it. We cannot ignore this issue, and it is crucial that we help young reach out for support so they do not suffer in silence."