There could be a link between smoking and schizophrenia, this is according to a research team from King's College London.
According to the team, smokers are more likely to develop the disorder and at a younger age.
Published in the Lancet Psychiatry, their analysis of 61 separate studies suggest nicotine in cigarette smoke may be altering the brain.
The team at King's looked at data involving 14,555 smokers and 273,162 non-smokers and found that 57% of people with psychosis were already smokers when they had their first psychotic episode.
Also, daily smokers were twice as likely to develop schizophrenia as non-smokers, and smokers developed schizophrenia a year earlier on average.
The argument is that if there is a higher rate of smoking before schizophrenia is diagnosed, then smoking is not simply a case of self-medication.
Experts said it was a "pretty strong case" but needed more research.
According to Dr James MacCabe, from the Institute of Psychiatry, Psychology and Neuroscience at King's, it is hoped that the study "open our eyes to the possibility that tobacco could be a causative agent in psychosis."
While most smokers do not develop schizophrenia, the researchers believe it is increasing the risk, and the overall incidence of the condition is one in every 100 people normally, which may be increased to two per 100 by smoking.
The researchers said nicotine altered levels of the brain chemical dopamine, which has already been implicated in the psychosis.
While pointing out that longer-term studies are needed to fully understand this potential link, Prof Michael Owen, the director of the Institute of Psychological Medicine at Cardiff University, said the researchers had made a "pretty strong case" that smoking may increase the risk of schizophrenia.