The association between psychosis and smoking tobacco has been observed for a long time. In England alone, 42% of cigarettes are smoked by people with mental health problems, and in the United States, 80% of those with schizophrenia smoke, compared to a national average of 20%. This has focused a debate around what came first, the smoking or the mental illness?
A new study, published in The Lancet, suggests that smoking tobacco might be a modest causal factor in psychosis. By completing a meta-analysis of 61 studies, the team found that 57% of people who were first diagnosed with psychosis were smokers, which the researchers claim implies that – at least in some cases – the smoking came first. But of course, there are underlying genetic factors that this fails to take into account, meaning that although a diagnosis may not have been made before they started smoking, they could have had a genetic predisposition for some form of psychosis.
It’s often been thought that people who suffer from schizophrenia are more likely to take up smoking to alleviate some of the distress caused by the condition, helping ease symptoms like impaired thinking and to counter the side effects of anti-psychotic drugs – in effect self-medicating. But that’s not what this study appears to show.
“These findings call into question the self-medication hypothesis by suggesting that smoking may have a causal role in psychosis,” a spokesperson from King's College London, where the study was conducted, said in a statement. “People with first episodes of psychosis were three times more likely to be smokers. The researchers also found that daily smokers developed psychotic illness around a year earlier than non-smokers.”
An excess of dopamine in the brain is thought to be the best explanation for psychoses like schizophrenia, as too much in the front of the brain can cause delusions and hallucinations. This is backed up by evidence that dopamine-blocking drugs help to soften these symptoms, while those drugs that increase the release of dopamine can worsen them. Nicotine causes the brain to release more dopamine.
“The fact is that it is very hard to prove causation without a randomised trial,” says Dr. Michael Owen from Cardiff University. The development of mental illness is a complex process and teasing apart all the different factors that might contribute is a vastly difficult task. The researchers stress that while there might be a link, the results are in no way conclusive and more work needs to be done.
“This new study combines previously published scientific data into a statistical analysis which found that smoking cigarettes appears to modestly increase the risk of developing schizophrenia in later life,” says Michael Bloomfield, a clinical lecturer in psychiatry at University College London, who wasn’t involved in the study.
“However, much more research is needed before scientists can say for certain that smoking definitely increases the risk of schizophrenia since it remains possible that people who would go on to develop schizophrenia are more likely to start smoking.”