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 antipsychotic 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: 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
Source: Jack Stonebridge – King’s College London
Image Credit: Image is in the public domain
Original Research: Full open access research for “Does tobacco use cause psychosis? Systematic review and meta-analysis” by Pedro Gurillo, MD, Dr Sameer Jauhar, MRCPsych, Robin M Murray, FRS, and James H MacCabe, FRCPsych in Lancet Psychiatry. Published online July 9 2015 doi:10.1016/S2215-0366(15)00152-2
Abstract
Does tobacco use cause psychosis? Systematic review and meta-analysis
Background
Although the association between psychotic illness and cigarette smoking is well known, the reasons are unclear why people with psychosis are more likely to smoke than are the general population. We aimed to test several hypotheses. First, that daily tobacco use is associated with an increased risk of psychotic illness in both case-control and prospective studies. Second, that smoking is associated with an earlier age at onset of psychotic illness. Finally, that an earlier age at initiation of smoking is associated with an increased risk of psychosis. We also aimed to derive an estimate of the prevalence of smoking in patients presenting with their first episode of psychosis.
Methods
We searched Embase, Medline, and PsycINFO and selected observational studies in which rates of smoking were reported in people with psychotic disorders, compared with controls. We calculated the weighted mean difference for age at onset of psychosis and age at initiation of smoking. For categorical outcomes, we calculated odds ratios from cross-sectional studies and risk ratios from prospective studies.
Findings
Of 3717 citations retrieved, 61 studies comprising 72 samples met inclusion criteria. The overall sample included 14 555 tobacco users and 273 162 non-users. The prevalence of smoking in patients presenting with their first episode of psychosis was 0·57 (95% CI 0·52–0·62; p<0·0001). In case-control studies, the overall odds ratio for the first episode of psychosis in smokers versus non-smokers was 3·22 (95% CI 1·63–6·33), with some evidence of publication bias (Egger's test p=0·018, Begg's test p=0·007). For prospective studies, we calculated an overall relative risk of new psychotic disorders in daily smokers versus non-smokers of 2·18 (95% CI 1·23–3·85). Daily smokers developed psychotic illness at an earlier age than did non-smokers (weighted mean difference −1·04 years, 95% CI −1·82 to −0·26). Those with psychosis started smoking at a non-significantly earlier age than did healthy controls (−0·44 years, 95% CI −1·21 to 0·34).
Interpretation
Daily tobacco use is associated with increased risk of psychosis and an earlier age at onset of psychotic illness. The possibility of a causal link between tobacco use and psychosis merits further examination.
“Does tobacco use cause psychosis? Systematic review and meta-analysis” by Pedro Gurillo, MD, Dr Sameer Jauhar, MRCPsych, Robin M Murray, FRS, and James H MacCabe, FRCPsych in Lancet Psychiatry. Published online July 9 2015 doi:10.1016/S2215-0366(15)00152-2
