Almost half of patients treated for a cannabis related mental disorder go on to develop a schizophrenic illness, a study has suggested.
Mental health campaigners are calling for an awareness campaign
The Danish study, in the British Journal of Psychiatry, found a third of them developed paranoid schizophrenia.
Cannabis has been linked with the condition, but few studies have looked at people with drug-induced symptoms.
The researchers said cannabis users showed signs of schizophrenic illness earlier than others with the condition.
Researchers looked at the incidence of schizophrenia-spectrum disorders, including schizophrenia, schizoptypal disorder and schizoaffective disorders.
The team from Aarhus Psychiatric Hospital obtained information on 535 patients treated for cannabis-induced psychotic symptoms from the Danish Psychiatric Central Register, who were then followed for three years.
They were then compared to 2,721 people treated for schizophrenia-spectrum disorders who had no history of cannabis-induced illness.
It was found that 44.5% of those with cannabis-induced psychotic symptoms went on to be diagnosed with schizophrenia-spectrum disorders.
Those who had used the drug also developed schizophrenia at an earlier age than people in the comparison group, with men showing symptoms at an average age of 24.6 years, compared with 30.7 years in the comparison group.
For women the difference was 28.9 years compared with 33.1 years.
The researchers, led by Mikkel Arendt, say the study did not show cannabis caused the development of schizophrenia - because it was not possible to control for other factors such as hereditary predisposition, other drug use and socio-economic status.
But they say the fact patients with cannabis-induced psychotic symptoms developed schizophrenia at a younger age than non-users suggests that cannabis use may hasten the progress of the illness.
A second study by researchers at the Zucker Hillside Hospital in New York looked at the brains of teenagers, comparing some who were heavy cannabis users with schizophrenic patients and healthy adolescents.
The team used a sophisticated scanning technique called diffusion tensor imaging (DTI) which measures the motion of water molecules in the brain which can indicate microscopic abnormalities.
They found similar abnormalities in an area of the brain linked to high level linguistic and auditory skill in cannabis users and those with schizophrenia.
They focussed on the arcuate fasciculus, a bundle of fibres connecting the Broca's area in the left frontal lobe and the Wernicke's area in the left temporal lobe.
The abnormalities were not seen in healthy teenagers.
Dr Mazar Ashtari, of the Albert Einstein College of Medicine who led the study, said: "Because this language/auditory pathway continues to develop during adolescence, it is most susceptible to the neurotoxins introduced into the body through marijuana use."
Dr Sanjiv Kumra, who also worked on the study, added: "These findings suggest that, in addition to interfering with normal brain development, heavy marijuana use in adolescents may also lead to an earlier onset of schizophrenia in individuals who are genetically predisposed to the disorder."
The research was presented to the annual meeting of the Radiological Society of North America.
Paul Corry, of the mental health charity Rethink, said: "This research reinforces our call for a public health information campaign about a drug which many young people still see as being risk free.
"We hope that the government, before too long, will recognise that need and take up advice from its advisory committee on the misuse of drugs, which we hope will report very soon."