A large review and analysis of the medical literature has found that people who have schizophrenia are three times more likely to smoke than those who do not. Other studies have found that people with psychotic conditions are more likely to smoke, but how smoking is associated with schizophrenia has not been studied. Based on their findings, the authors of this study propose that smoking could be a cause of psychotic illnesses.
The study was published in The Lancet Psychiatry and is what is called a meta-analysis, which means that the authors searched through the medical literature and combined the data from 61 studies of smoking and psychosis that involved 15,000 tobacco users and 273,000 non-users. The study found that 57% of people diagnosed with schizophrenia for the first time were smokers. The study also found that people who were diagnosed for the first time were three times more likely to smoke than those who did not have schizophrenia. Another finding is that daily smokers developed psychotic illness at an earlier age then did people who did not smoke.
Earlier studies of smoking and psychosis noted that people with schizophrenia and similar conditions may smoke to relieve anxiety or distress. They may also self-medicate with nicotine, which has some effects on the brain. One possibility is that heavy cigarette smoking increases levels in the brain of a chemical called dopamine. Dopamine is thought to play a role in schizophrenia.
"Excess dopamine is the best biological explanation we have for psychotic illnesses such as schizophrenia," said Robin M. Murray, one of the authors of the study, in a statement. Murray is professor of psychiatric research at King's College London. "It is possible that nicotine exposure, by increasing the release of dopamine, causes psychosis to develop."
The study is available online at http://www.thelancet.com/journals/lanpsy/article/PIIS2215-0366(15)00152-2/fulltext.