Cannabis use by children increases the risk of aggressive behaviour, but does not lead to them becoming withdrawn, a Dutch study says.
Cannabis is the most common drug offered to children in the UK.
Previous research on the drug has linked it to "internalised" problems such as depression.
But the British Journal of Psychiatry study of 5,000 children said it was more likely to cause external problems such as delinquency and aggression.
UK experts said the "jury was still out" about such an effect.
The findings comes as figures show more UK children are being exposed to cannabis.
Last year, a report by the Schools Health Education Unit, a government research team which has been tracking young people's experience of drugs since 1987, found over half of 14 and 15-year-olds had been offered cannabis, with one in four having taken it.
The latest study, by a team at the Trimbos Institute, a mental health research centre in the Netherlands, analysed the results of questionnaires filled in by 5,551 young people aged 12 to 16.
They found 17% had used the drug in the previous year.
Researchers found the strength of the link increased with higher use of cannabis.
However they found children who had used cannabis, but not in the previous year, were not at higher risk that those who had never used cannabis.
They also found more heavy cannabis users - children who used the drug 40 times a year or more - reported poorer school grades than those who did not use the drug.
And researchers added that, while no link was found between cannabis and mental health problems such as depression, that did not mean there was no connection between the two for some vulnerable people
Previous studies has found that long-term cannabis use can increase the risk of depression.
Report author Harald Wychgel said the findings could be even more acute in countries which did not have such a liberal approach to cannabis - the drug is not illegal in the Netherlands.
He said: "At young ages the use of cannabis is already strongly associated with delinquent and aggressive behaviour even after controlling for strong confounders such as alcohol and smoking."
Paul Corry, of mental health charity Rethink, said researchers were beginning to look at the links between aggression and mental health.
"There have been some studies which have found similar things, but I would say the jury is still out. What is not clear is where it is cause or effect. "Are these children already aggressive and the environment they are in increases the risk of them using cannabis?
"We have much more evidence that cannabis use is linked to feelings of anxiety and hallucinations."