Smoking at least ten cigarettes a day is linked to a higher risk of psychoses compared to non-smoking young people. The risk is also raised if the smoking starts before the age of 13. This has been shown in a study led by Academy Research Fellow, Professor Jouko Miettunen. The results were recently published in the journal Acta Psychiatrica Scandinavica.

"This was an extensive longitudinal study based on the general population. It revealed that daily and heavy smoking are independently linked to the subsequent risk of psychoses, even when accounting for previous psychotic experiences, the use of alcohol and drugs, substance abuse and the parents' history of psychoses. Smoking begun at an early age was a particularly significant risk factor. Based on the results, prevention of adolescent smoking is likely to have positive effects on the mental health of the population in later life," Miettunen says.

The aim of the study was to investigate whether young people's daily cigarette smoking is associated with a risk of psychoses, after accounting for several known, confounding factors, such as alcohol and drug use, the hereditary taint of psychoses and early symptoms of psychosis.

The research material comprised the 1986 birth cohort of Northern Finland and it originally included more than 9,000 people. 15-16-year-old members of the cohort were invited to participate in a follow-up study carried out in 2001-2002. The final sample included 6,081 subjects who answered questions on psychotic experiences and alcohol and drug use. The follow-up continued until the subjects had reached the age of 30.

The research team has also conducted a study on cannabis use, which has been published in The British Journal of Psychiatry. The study found that teenage cannabis use is associated with an increased risk of psychosis. It also showed that people who had used cannabis and had psychotic experiences early in life experienced more psychoses during the period of study.

"We found that young people who had used cannabis at least five times had a heightened risk of psychoses during the follow-up, even when accounting for previous psychotic experiences, use of alcohol and drugs and the parents' history of psychoses. Our findings are in line with current views of heavy cannabis use, particularly when begun at an early age, being linked to an increased risk of psychosis. Based on our results, it's very important that we take notice of cannabis-using young people who report symptoms of psychosis. If possible, we should strive to prevent early-stage cannabis use," says Antti Mustonen, Lic. Med.

The two studies were part of Jouko Miettunen's research project "Trends and interactions of risk factors in psychotic disorders -- Northern Finland birth cohort studies 1966 and 1986," which was funded by the Academy of Finland. The published articles are part of Antti Mustonen's forthcoming doctoral thesis on the link between alcohol and drug use and the risk of psychoses. In addition to researchers from the University of Oulu, the team included researchers from the University of Cambridge and the University of Queensland.