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Smoking tobacco linked to psychosis risk

Tobacco debate Smoking tobacco every day may increase the risk of developing psychosis, suggests a new study.

The research, published in The Lancet Psychiatry, challenges previous ideas about the link between psychosis and smoking.

It is well known that people with schizophrenia have a higher rate of smoking than the general population, but it is unclear why.

"While it's 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," says James MacCabe, a psychosis expert who co-led the research at King's College London's Institute of Psychiatry.

He adds, however, that tobacco was only one of many factors, including certain genetic, diet, lifestyle and other influences, raising a person's risk of developing schizophrenia.

Schizophrenia, a severe psychiatric disorder that affects around one in 100 people, typically begins in early adulthood. Its most common symptoms are disruptions in thinking and perception, and patients often have psychotic experiences.

Until now many doctors have followed a self-medication hypothesis whereby patients smoke to counteract the stressful symptoms of schizophrenia or the side-effects of antipsychotic medication.

McCabe and colleagues analysed data from 61 observational studies conducted around the world between 1980 and 2014 that included nearly 15,000 tobacco users and over 273,000 non-users.

They found that 57 per cent of people presenting with their first episode of psychosis were smokers.

People with a first episode of psychosis were three times more likely to be smokers than those in the control groups.

Daily smokers also developed psychotic illness about one year earlier than did non-smokers.

Activity in the brain's dopamine system might be one explanation of a possible causal link between smoking and psychosis, says study co-author Professor Robin Murray also of King's College London's Institute of Psychiatry.

"Excess dopamine is the best biological explanation we have for psychotic illnesses," says Murray.

"It's possible that nicotine exposure, by increasing the release of dopamine, causes psychosis to develop."

Previous studies, some by Murray, have also linked cannabis use to psychosis. But there is much debate about whether this is causal or whether there may be shared genes which predispose people to both cannabis use and schizophrenia.

McCabe says the new results on smoking suggest "it might even be possible that the real villain is tobacco, not cannabis" -- since cannabis users often combine the drug with tobacco.

It is possible that genes for psychosis may overlap with those for cigarette smoking, but much more research is needed to tease out any possible link, say other experts.

"There is hope that combining genetic analyses with longitudinal epidemiological study designs will help untangle some of these issues," says Professor Michael Owen, Director of the Institute of Psychological Medicine and Clinical Neurosciences, Cardiff University.

"It is very hard to prove causation without a randomised trial, but there are plenty of good reasons already for targeting public health measures very energetically at the mentally ill."

Related: Quitting smoking makes you happier