Cannabis is known to cause psychosis and schizophrenia. The authors of the analysis say are certain there is a modest effect caused by tobacco alone. Photograph: Chris Radburn/PA
Analysis of studies on smoking tobacco and psychosis, of which schizophrenia is most common type, suggests smoking may be causal factor in itself
Smoking cigarettes might increase people’s risk of psychosis, say researchers who believe tobacco as well as cannabis could play a part in causing schizophrenia.
It has long been recognised that people suffering from psychosis tend to smoke more than most of the population, but it has generally been assumed they are self-medicating. “Having psychosis is a very distressing thing – hearing voices, having delusions,” said Dr James MacCabe from King’s Institute of Psychiatry, Psychology and Neuroscience, one of the authors of a new study.
“The argument goes, why wouldn’t people smoke to alleviate the distress?” They might hope it would help with the symptoms and their impaired thinking processes and possibly counter the side-effects of antipsychotic drugs, he said.
But an analysis of a number of studies on smoking tobacco and psychosis, published in the Lancet Psychiatry journal, now shows that smoking may be a causal factor in itself.
The researchers found that daily smokers had an increased risk of psychosis. More than half - 57% - of people arriving at mental health services with their first episode were smokers, which is nearly three times the normal occurrence in the population. Smokers experienced psychosis one year earlier than non-smokers.
“We can’t say that we have proof that cigarette smoking causes schizophrenia,” said Sir Robin Murray, professor of psychiatric research at King’s. “Indeed it is very difficult to point to any particular factor and say it causes schizophrenia. It is a bit like heart disease – there are a number of risk factors. You inherit some vulnerability and … are exposed to various things which increase the risk to your life.”
Cannabis is known to cause psychosis and schizophrenia. The authors say they could not be certain that all the studies they looked at had completely accounted for cannabis use. However, they are certain there is a modest effect caused by tobacco alone.
There are biologically plausible reasons why smoking may be linked to psychosis. “Excess dopamine is the best biological explanation we have for psychotic illnesses such as schizophrenia,” said Murray. “It is possible that nicotine exposure, by increasing the release of dopamine, causes psychosis to develop.” A number of other drugs can stimulate dopamine production, including amphetamines, cocaine and cannabis.
There are also genetic clues – a small number of DNA sequences (called SNPs) are known to be implicated in both schizophrenia and smoking.
“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,” said MacCabe.
Prof Michael Owen, director of the Institute of Psychological Medicine and Clinical Neurosciences at Cardiff University, said this paper, together with a previous study carried out in Sweden, “make a pretty strong case that smoking is of causal relevance to schizophrenia”.
He hoped that further genetic investigation might help untangle the relationship. “The fact is that 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,” he said.