A team of researchers led by scientists from the University of Bristol in the UK has added weight to growing evidence that smoking can have a negative effect on mental health.
Rather than simply looking at whether the smokers had a genetic predisposition to depression or schizophrenia, the researchers used genetic data to examine cause-and-effect relationships with smoking,
"Individuals with mental illness are often overlooked in our efforts to reduce smoking prevalence, leading to health inequalities," the study's lead author, Robyn Wootton, said in a statement.
"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."
Scientists studied data from 462,690 individuals of European ancestry using an approach called Mendelian randomization.
This involves identifying genetic variations associated with a trait, such as depression or schizophrenia, and then testing for those variations against an exposure, like smoking, in a group of subjects.
This allows researchers to examine whether this relationship is causal.
The researchers concluded that while smoking increased the risk of depression and schizophrenia, people with depression and schizophrenia are also more likely to smoke. However, the authors noted that the relationship was weaker in those with schizophrenia.
The same team found that smoking also increases the risk of bipolar disorder, in another Mendelian randomization study published in September.