Background: There is a common perception that smoking generally helps people to manage stress, and may be a form of 'self‐medication' in people with mental health conditions. However, there are biologically plausible reasons why smoking may worsen mental health through neuroadaptations arising from chronic smoking, leading to frequent nicotine withdrawal symptoms (e.g. anxiety, depression, irritability), in which case smoking cessation may help to improve rather than worsen mental health.

Objectives: To examine the association between tobacco smoking cessation and change in mental health.

Main results: We included 102 studies representing over 169,500 participants. Sixty‐two of these were identified in the updated search for this review and 40 were included in the original version of the review.  Sixty‐three studies provided data on change in mental health, 10 were included in meta‐analyses of incidence of mental health disorders, and 31 were synthesised narratively. 

For all primary outcomes, smoking cessation was associated with an improvement in mental health symptoms compared with continuing to smoke: anxiety symptoms (SMD −0.28, 95% CI −0.43 to −0.13; 15 studies, 3141 participants; I2 = 69%; low‐certainty evidence); depression symptoms: (SMD −0.30, 95% CI −0.39 to −0.21; 34 studies, 7156 participants; I2 = 69%' very low‐certainty evidence);  mixed anxiety and depression symptoms (SMD −0.31, 95% CI −0.40 to −0.22; 8 studies, 2829 participants; I2 = 0%; moderate certainty evidence).  These findings were robust to preplanned sensitivity analyses, and subgroup analysis generally did not produce evidence of differences in the effect size among subpopulations or based on methodological characteristics. All studies were deemed to be at serious risk of bias due to possible time‐varying confounding, and three studies measuring depression symptoms were judged to be at critical risk of bias overall. There was also some evidence of funnel plot asymmetry. For these reasons, we rated our certainty in the estimates for anxiety as low, for depression as very low, and for mixed anxiety and depression as moderate.

For the secondary outcomes, smoking cessation was associated with an improvement in symptoms of stress (SMD −0.19, 95% CI −0.34 to −0.04; 4 studies, 1792 participants; I2 = 50%), positive affect (SMD 0.22, 95% CI 0.11 to 0.33; 13 studies, 4880 participants; I2 = 75%), and psychological quality of life (SMD 0.11, 95% CI 0.06 to 0.16; 19 studies, 18,034 participants; I2 = 42%). There was also evidence that smoking cessation was not associated with a reduction in social quality of life, with the confidence interval incorporating the possibility of a small improvement (SMD 0.03, 95% CI 0.00 to 0.06; 9 studies, 14,673 participants; I2 = 0%). The incidence of new mixed anxiety and depression was lower in people who stopped smoking compared with those who continued (OR 0.76, 95% CI 0.66 to 0.86; 3 studies, 8685 participants; I2 = 57%), as was the incidence of anxiety disorder (OR 0.61, 95% CI 0.34 to 1.12; 2 studies, 2293 participants; I2 = 46%). We deemed it inappropriate to present a pooled estimate for the incidence of new cases of clinical depression, as there was high statistical heterogeneity (I2 = 87%).

Authors' conclusions: Taken together, these data provide evidence that mental health does not worsen as a result of quitting smoking, and very low‐ to moderate‐certainty evidence that smoking cessation is associated with small to moderate improvements in mental health.  These improvements are seen in both unselected samples and in subpopulations, including people diagnosed with mental health conditions. Additional studies that use more advanced methods to overcome time‐varying confounding would strengthen the evidence in this area.