Smoking tobacco cigarettes could increase the risk of mental health problems like depression and schizophrenia, the research suggests.
It has long been known that smoking is more common among people with mental health problems. However, it is not clear whether smoking can be a factor in causing such problems or whether it is simply a form of self-medication among those who are already living in poor mental health.
Scientists say they have now solved the problem. "What we found out was that there was evidence of causal effects in both directions," said Dr. Robyn Wootton, the first author of the research, at the University of Bristol.
Wootton said that while the serious physical consequences of smoking were already known, the new research emphasized the importance of preventing people from starting the habit and helping smokers stop protecting their mental health, regardless of whether or not they have mental health problems.
"Of course, even if [smoking] worsens the risk of mental illness, we should also help people who have mental health problems to quit," he said.
Writing in the journal Psychological Medicine, Wootton and colleagues report how they compared the risk of developing depression or schizophrenia among people with and without genetic predisposition to smoking cigarettes. Since these genetic variants are randomly distributed throughout the population – and they have not changed by factors such as alcohol consumption, income, exercise or other health problems – the approach is a type of natural experiment which reduces the chances of any link being linked to other factors.
The team focused on 378 genetic variants that had previously been linked to the fact that people started smoking, as well as 126 genetic variants that the team discovered were linked to a higher score for the smoke of a lifetime – a measure that understand how much people smoke, for how long and if they stop.
Wootton and colleagues then used two separate genetic databases, one comprising thousands of individuals with schizophrenia and the other including thousands of individuals with major depression, to explore whether the risk of having such conditions was related to genetic variants of smoking.
The results reveal that both the beginning of smoking and the higher levels of smoking are linked to an increased risk of depression and schizophrenia. For example, an individual who smoked 20 cigarettes a day for 15 years but who did not smoke for 17 years was more than twice as likely to develop schizophrenia and almost twice as likely to develop depression than someone who never had smoked.
The team also tried to see if people with a genetic predisposition to depression or schizophrenia smoked more. Although they actually found such an effect, it was weaker than the opposite direction.
However, the study has limitations, including the fact that it focused on people of European origin.
Wootton said that it was necessary to explore exactly how smoking could increase the risk of schizophrenia and depression, but one possibility was that nicotine influences brain pathways related to mental health problems. It could be important, he added, because nicotine is also found in electronic cigarettes.
The use of cannabis could also help to explain the results, since high-strength cannabis has been previously suggested to increase the risk of mental health problems and those who smoke have an increased risk of cannabis dependence.
The dott. Ian Hamilton, an expert on addiction and mental health at the University of York, said: "While the physical damage caused by smoking is well known, this research indicates the risks to the mental health of tobacco use. This risk should be widespread, but particularly for school-age children who may be tempted to try smoking ”.