Given the amount of information about the negative consequences of smoking that is currently available, some might say that you would have to be mentally-ill to start smoking.  The good news is that stopping smoking has been associated with improvements in mental health (BMJ 2014; 348: g1151 dii 10.1136/bmj.g1151 -thanks to Mike Hart for this reference).  Free full-text BMJ article PDF

From Abstract:

Design Systematic review and meta-analysis of observational studies.

Eligibility criteria for selecting studies Longitudinal studies of adults that assessed mental health before smoking cessation and at least six weeks after cessation or baseline in healthy and clinical populations.

Results 26 studies that assessed mental health with questionnaires designed to measure anxiety, depression, mixed anxiety and depression, psychological quality of life, positive affect, and stress were included…. the standardised mean differences (95% confidence intervals) were anxiety −0.37 (95% confidence interval −0.70 to −0.03); depression −0.25 (−0.37 to −0.12); mixed anxiety and depression −0.31 (−0.47 to −0.14); stress −0.27 (−0.40 to −0.13). Both psychological quality of life and positive affect significantly increased between baseline and follow-up in quitters compared with continuing smokers 0.22 (0.09 to 0.36) and 0.40 (0.09 to 0.71), respectively). There was no evidence that the effect size differed between the general population and populations with physical or psychiatric disorders.

Conclusions Smoking cessation is associated with reduced depression, anxiety, and stress and improved positive mood and quality of life compared with continuing to smoke. The effect size seems as large for those with psychiatric disorders as those without. The effect sizes are equal or larger than those of antidepressant treatment for mood and anxiety disorders.

Figure 2 shows the relationship of the individual studies and the mental health outcomes.  In every study except one, there was improvement in those who quit smoking, though many of the studies had confidence limits that indicated that the results did not meet statistical significance.

Bottomline: This study indicates that quitting smoking can improve rather than worsen mental health.