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Tobacco smoking increases risk of depression and schizophrenia

Mendelian randomisation technique suggests cause-and-effect relationship exists between smoking, depression and schizophrenia

Ingrid Torjesen

Wednesday, 06 November 2019

Tobacco smoking may increase the risk of developing depression and schizophrenia, a study* published in A Psychological Medicine has found.

It is well known that smoking is much more common amongst people with mental illness, especially those with depression and schizophrenia, however it has been unclear whether this is a cause-and-effect relationship, and if so in which direction.

Researchers from the Tobacco and Alcohol Research Group (TARG) at the University of Bristol accessed UK Biobank data from 462,690 individuals of European ancestry, of whom 8% were current smokers and 22% former smokers.

They used Mendelian randomisation, a technique which uses genetic variants associated with an exposure (e.g. smoking) to support stronger conclusions about cause-and-effect relationships, and found evidence that tobacco smoking increased risk of depression and schizophrenia, but also that depression and schizophrenia increase the likelihood of smoking (although the evidence was weaker in this direction for schizophrenia).

The researches concluded: “These findings suggest that the association between smoking, schizophrenia and depression is due, at least in part, to a causal effect of smoking, providing further evidence for the detrimental consequences of smoking on mental health.”

The study adds to a growing body of work suggesting that smoking can have adverse effects on mental health. The same group published a similar study in British Journal of Psychiatry earlier this year in collaboration with the University of Amsterdam, showing evidence that tobacco smoking increases the risk of bipolar disorder.

Dr Robyn Wootton, senior research associate in the School of Experimental Psychology and the study's lead author, said: "Individuals with mental illness are often overlooked in our efforts to reduce smoking prevalence, leading to health inequalities. Our work shows that we should be making every effort to prevent smoking initiation and encourage smoking cessation because of the consequences to mental health as well as physical health."

*Wootton RE, Richmond RC, Stuijfzand BG, et al. Evidence for causal effects of lifetime smoking on risk for depression and schizophrenia: a Mendelian randomisation study. A Psychological Medicine. Published online by Cambridge University Press: 06 November 2019. DOI: 10.1017/S0033291719002678

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