Mental illness could be yet another fallout of smoking tobacco, besides cancer and strokes, says research from King's College London.
Daily smokers turned psychotic faster than non-smokers in data analysed from 61 observational studies involving 15,000 tobacco users and 273,000 non-smokers.
Researchers at King's Institute of Psychiatry, Psychology & Neuroscience (IoPPN) analysed rates of smoking in people presenting with their first episode of psychosis and found that 57% of these individuals were smokers.
They were three times more likely to be smokers than those in the control groups.
Psychosis refers to symptoms of illnesses where patients exhibit loss of contact with reality. There are two types of psychiatric disorder that produce psychotic symptoms: schizophrenia, and mood disorders such as bipolar disorder.
People with certain mental illnesses have been seen to resort to increased smoking for relief from boredom or distress, or self-medicating to counter the side-effects of anti-psychotic medication.
But the King's study found increased cases of smoking before psychosis had developed.
However, more research will be needed to establish the connection. Some of the studies analysed also included people who consumed substances other than tobacco, such as cannabis, which may have had an impact on the results.
Dr James MacCabe, clinical senior lecturer in Psychosis Studies at the IoPPN, King's College London, said: "While it is always hard to determine the direction of causality, 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."
The brain's dopamine system could possibly explain the causal link between smoking and psychosis. Nicotine exposure could be releasing excess dopamine leading to psychosis, said Professor Sir Robin Murray, professor of Psychiatric Research.