A new study shows smoking can triple the chances of a person developing psychosis and may trigger serious mental illnesses such as schizophrenia.
It has long been believed people with psychotic mental illnesses are more likely to smoke due to non-casual factors, such as stress relief or self-medication, the Huffington Post reports.
The new study, published in The Lancet Psychiatry medical journal, shows scientists now believe tobacco, alongside genetic and environmental influences, may be responsible for psychosis.
"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," says a member of the team from King's College London Dr James MacCabe.
The study found 57 percent of patients treated for a first-time psychosis were smokers and smokers were three times more likely to consume tobacco than individuals without severe mental illness.
It also showed daily smokers become psychotic around one year earlier than non-smokers.
Scientists acknowledged in the study that causalities were hard to prove and one theory is a possible link between smoking and excess dopamine, which is a brain chemical that plays a role in transmitting nerve signals.
"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," says Professor Sir Robin Murray from the Institute of Psychiatry, Psychology and Neuroscience at King's College.
There has been a proven link between cannabis use and psychosis in genetically vulnerable people.
Dr Sameer Jauhar, another member of the King's College team, says longer-term studies are needed to investigate the relationship between daily smoking, sporadic smoking, nicotine dependence and the development of psychotic disorders.
Dr Michael Bloomfield, clinical lecturer in psychiatry at University College London, says the new study found that smoking cigarettes appears to modestly increase the risk of developing schizophrenia in later life.
"Regardless of these findings, there is overwhelming evidence that nicotine use through tobacco smoking is one of the most dangerous drug problems in the world. Anyone who needs help in stopping smoking should speak with their doctor."
Data from 61 observational studies involving almost 15,000 tobacco users and 273,000 non-users was analysed for the study.