A new study, published in the journal Psychological Medicine, has revealed that smokers may be at greater risk of developing depression and schizophrenia. The study conducted by a team of scientists from the UK’s University of Bristol has re-emphasized that smoking can adversely affect mental health.

Instead of focusing on whether the smokers had a genetic predisposition to depression or schizophrenia, the scientists used genetic data to observe cause-and-effect relationships with smoking. Robyn Wootton, the study’s lead author, said in a statement, “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.”

Data from 462,690 people of European ancestry was examined for this study using an approach known as Mendelian randomization. The latter involves identifying genetic variations associated with a trait, such as depression or schizophrenia, and then testing for those variations against an exposure, such as smoking, in a group of subjects. This enables scientists to examine whether this relationship is causal or not.

The scientists concluded that whereas smoking increased the risk of depression and schizophrenia, individuals with depression and schizophrenia are also more liable to smoke. The authors, however, noted that the association was weaker in those with schizophrenia. The team further found that smoking also increases the risk of bipolar disorder, in another Mendelian randomization study published in September 2019. The scientists recommended that psychiatric hospitals be made smoke-free to prevent harmful effects on mental health.

On the other hand, a retired consultant psychiatrist and honorary professor at University College London and Queen Mary University of London, David Curtis, although not involved in the study, interprets its results differently to the authors.

Curtis says he doesn’t think it’s plausible that smoking acts directly on the brain to increase schizophrenia risk, and the results likely show the effect of mothers’ smoking when they were pregnant—a risk factor for schizophrenia. “So what we are likely seeing is that the mothers of people with schizophrenia were at a higher genetic risk of smoking, smoked during pregnancy and thereby increased the risk of schizophrenia developing in their children,” Curtis said in a statement. “And of course they would then also pass on an increased genetic risk of smoking to those children, which is what this study is picking up,” he added.

According to a Lancet Psychiatry Commission report published in July 2019, people with mental illness die up to 20 years earlier than the general population. Another study published in the journal JAMA Psychiatry in January 2018 found that the use of either marijuana or cigarettes is related to a greater risk of psychotic-like experiences in teenagers. Psychosis  is a mental condition characterized by a disconnection from reality causing hallucinations or delusions.

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