The researchers say the effect sizes are equal or larger than those of antidepressant treatment for mood and anxiety disorders.
It is well known that stopping smoking substantially reduces major health risks, such as the development of cancers, cardiovascular and respiratory diseases. But the association between smoking and mental health is less clear cut.
Many smokers want to stop but continue smoking as they believe smoking has mental health benefits. And health professionals are sometimes reluctant to deal with smoking in people with mental disorders in case stopping smoking worsens their mental health.
So researchers from the universities of Birmingham, Oxford, and King's College London set out to investigate changes in mental health after smoking cessation compared with continuing to smoke.
They analysed the results of 26 studies of adults that assessed mental health before smoking cessation and at least six weeks after cessation in the general population and clinical populations (patients with chronic psychiatric and/or physical conditions).
Differences in study design and quality were taken into account to minimise bias.
Measures of mental health included anxiety, depression, positivity, psychological quality of life, and stress. Participants had an average age of 44, smoked around 20 cigarettes a day, and were followed up for an average of six months.
The research team found consistent evidence that stopping smoking is associated with improvements in depression, anxiety, stress, psychological quality of life, and positivity compared with continuing to smoke.
The strength of association was similar for both the general population and clinical populations, including those with mental health disorders. And there was no evidence that study differences could have skewed the results.
Although observational data can never prove causality, "smokers can be reassured that stopping smoking is associated with mental health benefits," say the authors.
"This could overcome barriers that clinicians have toward intervening with smokers with mental health problems," they add. "Furthermore, challenging the widely held assumption that smoking has mental health benefits could motivate smokers to stop."
More information: www.bmj.com/cgi/doi/10.1136/bmj.g1151