London - Smoking tobacco may increase the risk of developing schizophrenia and other serious mental illnesses, scientists have suggested.
In a landmark study published today, British researchers highlight statistical trends which suggest cigarettes can increase the chance of someone being diagnosed with psychosis.
They think nicotine interferes with the way the brain responds to pleasure and reward, which may trigger mental disorders.
Doctors have highlighted in the past the fact that people with psychotic mental illnesses are more likely to be smokers. But until now experts have assumed this is because sufferers are smoking to deal with their distress. The study, in the respected Lancet Psychiatry medical journal, suggests for the first time the link may actually be causal.
The authors used data from 290 000 people to conclude that those with psychosis are more likely to smoke because their habit – alongside genetic and environmental influences - had caused their problem.
Dr James MacCabe, a member of the team from 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.”
The researchers analysed data from 61 studies involving 15 000 smokers and 273 000 non-smokers. They found that 57 percent of people treated for a first episode of psychosis were smokers.
Psychotic patients were three times more likely to smoke than people without mental illness. The study also showed that, on average, daily smokers became psychotic about a year earlier than non-smokers.
The scientists admitted that they had only shown a statistical link - and had proved nothing about what in tobacco might actually contribute towards mental illness. But, speculating on possible causes, they said there may be link between smoking and dopamine, a chemical that plays a role in the way the brain experiences emotion.
Professor Sir Robin Murray, of the Institute of Psychiatry at King’s College, 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.”
Experts welcomed the findings, although they stressed the link would be difficult to prove.
Professor Sir Michael Owen of Cardiff University said that within the constraints of its method the study “makes a pretty strong case”.
But he added: “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.”
Daily Mail