November 06, 2019
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Smoking may increase risk for schizophrenia and depression

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Robyn Wootton

Tobacco smoking may increase risk for depression and schizophrenia, according to results published in Psychological Medicine.

“For individuals already suffering with a mental illness, it is a commonly held belief that smoking is a form of 'self-medication' and therefore individuals with mental health problems are often not helped to quit as much as they should be,” Robyn Wootton, PhD, senior research associate at University of Bristol’s School of Psychological Science in England, told Healio Psychiatry. “We already know that smoking results in a much higher proportion of smoking-related deaths/illnesses in these populations so we should be trying to help individuals to quit anyway, but this adds further weight to the argument that smoking is exacerbating mental illness.”

To explore the bi-directional effects of smoking on depression and schizophrenia, Wootton and colleagues utilized Mendelian randomization — an analytic approach that uses genetic variants associated with an exposure, such as smoking, to strengthen conclusions about cause-and-effect relationships. They analyzed genome-wide association studies (GWAS) from the Sequencing Consortium of Alcohol and Nicotine use and conducted their own GWAS of lifetime smoking behavior in a sample of 462,690 individuals from the United Kingdom Biobank.

The researchers found “strong evidence” that smoking is a risk factor for both schizophrenia (OR = 2.27; 95% CI, 1.67-3.08) and depression (OR = 1.99; 95% CI, 1.71-2.32), and these results were consistent across both smoking initiation and lifetime smoking. Further, they noted that genetic liability to depression increases smoking (beta = 0.091; 95% CI, 0.027-0.155), but evidence was mixed for schizophrenia (beta = 0.022; 95% CI, 0.005-0.038) and very weak for an effect on smoking initiation.

“For individuals without mental illness, of course we already know that they should not take up smoking because of the detrimental effects on physical health,” Wootton said. “But this study shows that smoking also has detrimental effects on mental health, further stressing the importance that individuals should not smoke.” – by Joe Gramigna

Disclosures: The authors report no relevant financial disclosures.