Smoking cigarettes associated with increased risk of psychosis

A new study by researchers at King's College London suggests that smoking tobacco is associated with an increased risk of developing psychosis.

People who suffer from psychosis are three times more likely to smoke cigarettes than healthy controls, according to the meta-analysis published today in Lancet Psychiatry.

Although the association between smoking cigarettes and psychosis - particularly schizophrenia - has been acknowledged before, scant attention has been directed towards the possibility that cigarettes themselves may increase the risk of psychosis.

Instead, in the past, various different explanations have been proposed for why people with psychosis are more likely to smoke compared with the rest of the population. These include relief from boredom or distress, and self-medication, whereby smoking counteracts the negative symptoms of schizophrenia or the side-effects of anti-psychotic medication.

However, if this were the case, researchers would expect the rates of smoking to increase only after an individual had developed psychosis. 

Researchers at King's Institute of Psychiatry, Psychology & Neuroscience (IoPPN) conducted a meta-analysis of 61 observational studies comprising almost 15,000 tobacco users and 273,000 non users. They analysed rates of smoking in people presenting with their first episode of psychosis and found that 57 per cent of these individuals were smokers.

People with a first episode of psychosis were three times more likely to be smokers than those in the control groups. The researchers also found that daily smokers developed psychotic illness around a year earlier than non-smokers.

According to the authors, these findings call into question the self-medication hypothesis by suggesting that smoking may have a causal role in psychosis, alongside other genetic and environmental factors.

However, they acknowledge that despite finding an association between smoking and psychosis, the direction of causality is difficult to determine. In addition, very few studies included in the meta-analysis controlled for the consumption of 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.'

It is thought that activity within the brain's dopamine system might be one explanation of a possible causal link between smoking and psychosis.

Professor Sir Robin Murray, Professor of Psychiatric Research at the IoPPN, King's College London, said: '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.

Dr Sameer Jauhar, Research Fellow at the IoPPN, King's College London, said: 'Longer-term studies are required to investigate the relationship between daily smoking, sporadic smoking, nicotine dependence and the development of psychotic disorders.

'In view of the clear benefits of smoking cessation programmes in this population every effort should be made to implement change in smoking habits in this group of patients.'

Funding for this research was provided by the NIHR Biomedical Research Centre at the South London and Maudsley NHS Foundation Trust (SLaM) and King's College London.