CANNABIS AND SCHIZOPHRENIA: THE LINKS

1972: Study of US servicemen finds widespread cannabis smoking contributed to a five-fold increase in schizophrenia between 1968-1971.

1974: Researchers find cannabis could trigger latent schizophrenia, intensify symptoms and cause the condition even where it would not otherwise occur.

1981: Study of 16-year olds in Sweden finds that heavy users are three times as likely to have mental-health problems by 27 years old - a 10% risk.

1986: US National Institute for Drug Abuse finds that schizophrenics who smoked cannabis have worse symptoms, more relapses, need more help and are less responsive to treatment.

1987: Study of 45,000 swedish military conscripts finds that those who smoked cannabis 50 times or more by age 18 are six times more likely to suffer schizophrenia later.

2002: A 26-year study by King's College London shows that people who began smoking it in early teens faced four-fold increase in risk of schizophrenia as adults, and established that the drug directly triggered the disease.

2003: US scientists show that those using cannabis before the age of 17 are up to five times more likely to have drug and alcohol problems later in life.

2005: Study led by the Institute of Psychiatry in London finds that one in four cannabis users are genetically predisposed to suffer mental ilness due to the drug.

2005: Danish researchers find almost 50% of cannabis users who suffer a drug-induced psychotic episode go on to develop long-term schozophrenia.

2007: The Institute of Psychiatry finds that cannabis' most powerful active ingredient reduced activity in a part of the brain which helps to keep people sane.

2007: Study by the universities of Cardiff, Bristol, Cambridge and Imperial College London finds that heavy users of cannabis are more than twice as likely to suffer mental illness.