These results provide evidence of a strong, statistically independent, dose-related association between smoking and completed suicide from a large, well-controlled, population-representative cohort with the longest follow-up to date and the first to report a within-twin-pair analysis of smoking and suicide. They support and extend prior reports that active but not former smoking is associated with suicide (Iwasaki et al. 2005; Li et al. 2012), with heavy smoking associated with greater suicide risk than lighter smoking (Iwasaki et al. 2005). They suggest that tobacco smoking is an environmental factor that is possibly etiologically associated with suicide. The findings are intriguing in light of a recent meta-analysis reporting smoking abstinence, compared with continued smoking, associated with reduced depression, anxiety, and stress and increased positive mood and quality of life, with effect sizes comparable with antidepressant treatment (Taylor et al. 2014). The results support the hypothesis that exposure to tobacco smoke is a risk for suicide and that reduced risk of suicide is yet another health benefit of smoking cessation. Study of chronic neurobiological effects of tobacco smoke constituents may aid the effort
