Nonaffective psychoses refer to types of psychosis that are not related to emotions or moods. Examples of nonaffective psychosis include schizophrenia and delusional disorders.

Researchers headed by Dr. John McGrath from the Queensland Brain Institute at the University of Queensland performed the study.

They studied 3,081 young adults who were born at a Brisbane, Australia hospital, with an average age of about 20 years, and, thus, were born between 1981 and 1984. The adults were taking part in the Mater-University Study of Pregnancy.

The result of their study is published in the journal Archives of General Psychiatry under the title 'Association Between Cannabis Use and Psychosis-Related Outcomes Using Sibling Pair Analysis in a Cohort of Young Adults' (2010;67(5); doi:10.1001/archgenpsychiatry.2010.6}.

Its authors are: John McGrath, Joy Welham, James Scott, Daniel Varghese, Louisa Degenhardt, Mohammad Reza Hayatbakhsh, Rosa Alati, Gail M. Williams, William Bor, and Jake M. Najman.

The young people were asked questions about their use of cannabis. The researchers also assessed the subjects as to the presence or absence of psychoses; specifically whether or not they had delusional behavior and/or hallucinations.

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The Australian researchers found that over 14% of the young people had used cannabis for six years or more, 16% had smoked for between four and five year, and almost 18% had smoked cannabis for three years or less.

Of the 3,081 young adults, 65 of them were diagnosed as suffering from schizophrenia while 233 of them had some types of hallucinations.

Specifically, of those subjects that had used cannabis for an extended period, they were diagnosed as two times as likely to have hallucinations and four times as likely to show delusional behavior, when compared to the subjects that had not used cannabis.

The February 28, 2010 AFP article 'Long-term cannabis use increases risk of mental ills: study' said the researchers stated in their paper that, "There were significant linear trends between the exposure variable and all three psychosis-related measures: the longer the duration since first cannabis use, the higher the risk of the adverse outcomes.'

The study continued with this statement: "Those individuals who ... had isolated psychotic symptoms were more likely to commence cannabis use, which could then contribute to an increased risk of conversion to a non-affective psychotic disorder.' [AFP]

Dr. John McGrath, one of the researchers of the study, stated: "Compared with those who had never used cannabis, young adults who had six or more years since first use of cannabis were twice as likely to develop a non-affective psychosis and were four times as likely to have high scores on the Peters et al Delusions Inventory - a measure of delusion.' [AFP]

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Dr. McGrath added, "There was a 'dose-response' relationship between the variables of interest - the longer the duration since first cannabis use, the higher the risk of psychosis-related outcomes."

And, their paper stated, "Apart from the implications for policy-makers and health-planners, we hope our findings will encourage further clinical and animal model-based research to unravel the mechanisms linking cannabis use and psychosis.'

They concluded, as stated in the abstract to their paper: 'Early cannabis use is associated with psychosis-related outcomes in young adults'¦. This study provides further support for the hypothesis that early cannabis use is a risk-modifying factor for psychosis-related outcomes in young adults.'

Further information on cannabis use and its risks are found at the NHS website The Dangers of Cannabis.