Cigarette smoking is very common among people with mental illness, especially schizophrenia, a severe disorder marked by delusional thoughts and hallucinations. Sameer Jauhar, a psychiatric researcher at King’s College London, noticed the relationship.
“As a psychiatrist, it’s always something you think about, because you always see patients with psychotic illnesses smoking," he said. "People always generally put it down to self-medication.”
Jauhar made his comments in a podcast posted by the online journal The Lancet Psychiatry, which published his study of the relationship.
Findings by the King’s College team offer the intriguing notion that cigarette smoking may in some way increase the risk of developing schizophrenia.
The researchers analyzed 61 studies worldwide and found the tobacco habit is three times more common among schizophrenic patients, coming to treatment for the first time, compared with the general population.
Approximately one-quarter of people who live in Western countries are cigarette smokers.
James MacCabe, a senior study co-author and doctor of psychiatric research at King’s College, said the fact that more than half of schizophrenics smoke is not surprising. What the finding does challenge, he said, is the belief that people who suffer from psychosis are smoking to ease their symptoms.
“If the explanation was self-medication, when people started to develop schizophrenia they would be no more likely to smoke than anybody else, and then the smoking would develop in response to the symptoms,” he said.
But the authors found people with schizophrenia started smoking at a younger age than other smokers, before psychotic symptoms appeared.
There is a suggestion, but no findings to prove it, that nicotine stimulates production of the brain chemical dopamine. Dopamine helps control the brain's reward and pleasure centers. Increased levels have been observed in patients with psychosis, and medications to treat schizophrenia aim to reduce dopamine activity.
If a definitive link is made between smoking and the development of schizophrenia, MacCabe said, patients at risk for the disorder — because of family history or substance abuse — might be well advised to kick the habit.
“There are many reasons why we should try to reduce rates of smoking in population for physical health reasons," he said. "But this suggests that possibly doing so would actually benefit people’s medical [mental] health.”
The authors are calling for more in-depth studies investigating the role of sporadic smoking and nicotine addiction in the development of psychotic disorders.