Teenage cannabis users are more likely to suffer psychotic symptoms that raise the risk of full-blown mental illness in later life, claim researchers.
Adolescent drug takers interviewed for the largest study of its kind reported experiencing at least three symptoms indicating a risk of psychosis.
These included feeling like something strange or inexplicable was taking place, suspecting they were being influenced or followed and difficulty in controlling the speed of thoughts.
Frequent drug users were more at risk but even youngsters dabbling at an early stage were vulnerable.
The researchers also found taking cannabis in adolescence creates a greater risk of developing schizophrenia than using it when older.
Their findings will bolster the Government's decision last month to restore cannabis to the harsher Class B status against the advice of its own scientists.
But it will also increase dismay that the change makes no difference to the way under-18s caught with cannabis are dealt with by the police. They still only face prosecution if caught with the drug three times.
British psychiatrists have consistently expressed concerns that the drug appears to accelerate the onset of full-blown mental illness, raising fears of an epidemic of mental health problems among users in their 20s and 30s.
More than 6,000 youngsters aged 15 and 16 were involved in the research in Finland, published today in the British Journal of Psychiatry. Experts assessed which were potentially at risk of developing a psychotic disorder.
More than 5 per cent of the teenagers reported that they had used cannabis once or more; and one in 100 had used cannabis more than five times. Girls were more likely to take the drug than boys.
It was found that those who had used cannabis had a higher average number of symptoms indicating a risk of psychosis.
Dr Jouko Miettunen, who led the research at the University of Oulu in Finland, said the findings were significant because they demonstrated a link with psychosis at a very early stage of cannabis use.
He said: 'These teenagers are likely to be vulnerable to the mental effects, which means they are probably vulnerable to developing psychosis at some point.'
Dr Miettunen said psychotic symptoms were much more common than actual cases of diagnosed mental illness but the number being reported was a cause for concern.
Campaigner Debra Bell, who formed the group Talking About Cannabis after her son William started taking cannabis at 14 and ended up stealing from his parents to feed his habit, said the research was more proof of the dangers.
She said 'All parents of children who smoke cannabis know the drug can cause serious mental disturbances. This study from Finland is very interesting but why aren't we doing this research here or doing something about the findings?
'It shows the problems begin at a very early age yet teenagers tell their parents it's a safe drug. Unfortunately, like my son, they can turn around and say it won't do them any harm because the Government downgraded it, despite the evidence showing we are storing up huge problems for the future.'