Researchers say that smoking at least 10 cigarettes daily my increase a person's risk of psychosis.
The tobacco study has now been published in the journal Acta Psychiatrica Scandinavica, and the marijuana study — which was conducted by the same team — has now been published in The British Journal of Psychiatry.
However, the precise reasons why people who experience psychosis are more likely to smoke are not clear.
Some scientists think that smoking might act as a kind of "self-medication" — that is, people with psychosis might find that smoking relieves their symptoms, perhaps due to some unidentified neurological mechanism.
Or, smoking might help to make people who have psychosis less bored or stressed, which could also alleviate symptoms.
Recently, studies have started to investigate whether smoking itself might increase a person's risk of psychosis. Although much research has looked at whether smoking marijuana might contribute to an increased risk of psychosis, comparatively few papers have applied the same investigative approach to tobacco.
'Self-medication' theory questioned
A 2015 systematic review and meta-analysis that was published in The Lancet examined this issue. Its authors reported that smoking tobacco every single day was linked with both increased risk of psychosis and earlier age of onset of psychotic disorder.
In this latest systematic review, people having a first episode of psychosis were three times more likely to be smokers than nonsmokers.
Based on their findings, the authors questioned the "self-medication" theory and proposed instead that nicotine may be having an effect that creates the conditions for psychosis, possibly on the dopamine system.
Dopamine is the neurotransmitter that helps to control the brain's "pleasure and reward centers." Scientists now know that smoking feels pleasurable because nicotine causes dopamine to be released into the brain.
Part of the reason why the authors of The Lancet study believe that the dopamine system may play a role in driving the link between daily smoking and psychosis is because studies have shown that smokers are less likely to get Parkinson's disease.
While Parkinson's disease is characterized by a lack of dopamine, schizophrenia is thought to be "the opposite of Parkinson's," in that some scientists believe that its symptoms are caused by an excess of dopamine.
Psychosis risk higher in daily smokers
In the tobacco study, the researchers analyzed data from 6,081 individuals who were part of the 1986 birth cohort of Northern Finland. Participants who were 15–16 years old in 1986 answered questions on psychotic experiences and whether they used drugs or alcohol. They were then followed until they reached the age of 30.
The team found that smoking heavily or daily was linked with increased risk of psychosis.
Individuals who smoked 10 or more cigarettes per day were more likely to experience psychosis than people who did not smoke. Furthermore, people who began smoking before the age of 13 were also found to be at increased risk of psychosis.
Even when the researchers took into account whether the people in the study used alcohol or drugs or had a family history of psychosis, the link between smoking and psychosis was still significant.
"Based on the results, prevention of adolescent smoking is likely to have positive effects on the mental health of the population in later life," concludes study author Jouko Miettunen.
Marijauna use linked to psychosis risk
In the study of marijuana, the team found an increased risk of psychosis among teenage users.
"We found that young people who had used cannabis at least five times had a heightened risk of psychoses during the follow-up, even when accounting for previous psychotic experiences, use of alcohol and drugs, and the parents' history of psychoses," notes study co-author Antti Mustonen.
"Our findings are in line with current views of heavy cannabis use, particularly when begun at an early age, being linked to an increased risk of psychosis," he adds.
"Based on our results, it's very important that we take notice of cannabis-using young people who report symptoms of psychosis. If possible, we should strive to prevent early-stage cannabis use."