A meta-analysis published in the journal Lancet Psychiatry reports that people who smoke are more than three times more likely to suffer from psychosis, compared with nonsmokers.
Previous studies have reported a link between smoking cigarettes and psychosis. However, few studies investigated cigarettes as a direct driver of this association.
Instead, researchers had hypothesized that people with psychosis are more likely to smoke because they may find that smoking counteracts side effects from schizophrenia medication or negative schizophrenia symptoms.
One problem with this hypothesis, however, is that rates of smoking would only increase after someone has developed psychosis for this to be the case.
The new meta-analysis, conducted by researchers at King's College London in the UK, assessed evidence from 61 observational studies, which involved 15,000 tobacco users and 273,000 nonusers overall.
The analysis shows that 57% of people presenting with a first episode of psychosis were smokers - this means they were three times more likely to be smokers than healthy, nonsmoking study participants.
Daily smokers were also found to develop psychotic illness around 1 year earlier on average than nonsmokers.
These findings shed doubt on the theory that an association between smoking and psychosis exists because people with psychosis use cigarettes to self-medicate, claim the authors.
"While it is always hard to determine the direction of causality," says Dr. James MacCabe, clinical senior lecturer in Psychosis Studies at the King's Institute of Psychiatry, Psychology & Neuroscience (IoPPN), "our findings indicate that smoking should be taken seriously as a possible risk factor for developing psychosis, and not dismissed simply as a consequence of the illness."
Because very few of the studies in the meta-analysis took into account consumption of substances other than tobacco, it was difficult for the King's team to rule out other factors that may contribute to the association between smoking and psychosis.
Nicotine, psychosis and dopamine
The researchers do, however, propose another hypothesis that could explain the association. Sir Robin Murray, professor of Psychiatric Research at the IoPPN, points to the brain's dopamine system:
"Excess dopamine is the best biological explanation we have for psychotic illnesses such as schizophrenia. It is possible that nicotine exposure, by increasing the release of dopamine, causes psychosis to develop."
"Longer-term studies are required to investigate the relationship between daily smoking, sporadic smoking, nicotine dependence and the development of psychotic disorders," says IoPPN research fellow Dr. Sameer Jauhar.
"In view of the clear benefits of smoking cessation programs in this population every effort should be made to implement change in smoking habits in this group of patients."
A 2014 study conducted by researchers from the Washington University School of Medicine in St. Louis, MO, found that people with severe mental illness such as schizophrenia or bipolar disorderhave a higher risk for substance abuse - especially cigarette smoking.
In that study, people with severe mental illness were:
- 4 times more likely to be heavy alcohol users (four or more drinks per day)
- 3.5 times more likely to use marijuana regularly (21 times per year)
- 4.6 times more likely to use other drugs at least 10 times in their lives
- 5.1 times more likely to be daily smokers.