People who suffer from psychosis are about three times more likely to be smokers, but scientists have long scratched their heads over which one leads to the other.
On Friday, research published in The Lancet Psychiatry suggested daily tobacco use, already known to cause cancer and stroke, may be also be a contributor to mental illness—not necessarily result of it.
Analysing data from 61 studies conducted around the world between 1980 and 2014, a team found that 57 percent of people first diagnosed with psychosis were smokers.
The studies contained data on nearly 15,000 smokers and 273,000 non-smokers, some of whom were diagnosed with psychotic illnesses like schizophrenia.
"People with first episodes of psychosis were three times more likely to be smokers," said a statement from King's College London's Department of Psychosis Studies, which took part in the meta-analysis.
"The researchers also found that daily smokers developed psychotic illness around a year earlier than non-smokers."
It has long been hypothesised that higher smoking rates among psychosis sufferers could be explained by people seeking relief from boredom or distress, or self-medicating against the symptoms or side-effects of antipsychotic medication.
But if this were so, researchers would expect smoking rates to increase only after people had developed psychosis.
"These findings call into question the self-medication hypothesis by suggesting that smoking may have a causal role in psychosis," said the statement.
The team stressed they had not conclusively proven that smoking causes psychosis, saying further research must be done.
But the results did suggest that smoking "should be taken seriously as a possible risk factor for developing psychosis and not dismissed simply as a consequence of the illness," they wrote.
The researchers theorised that changes in the brain's dopamine system may explain the association.
Dopamine is a chemical messenger that helps control the brain's reward and pleasure centres.
"Excess dopamine is the best biological explanation we have for psychotic illnesses such as schizophrenia," said King's College psychiatric professor Robin Murray.
"It is possible that nicotine exposure, by increasing the release of dopamine, causes psychosis to develop."
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