Objective
Recently, there is an increase in evidence that both smoking and mental health are continuously increasing among adolescents. This systematic review elucidates the research on evidence of the direction of association and risk factors influencing this relationship between smoking and depression. We also highlight the recent studies on the effects of electronic cigarettes and the development of the association between depression and smoking.      

Method 
A literature search was conducted on databases including PubMed, OVID Medline, EMBASE, and PsychInfo and in relevant neurology and psychiatry journals. Terms used for electronic searches included smoking, tobacco, cigarettes; depression; adolescent, youth; direction. The relevant information was then utilized to synthesize the association between smoking and depression among adolescent population. 

Results
The initial search of the databases yielded 2738 related articles. After screening and cross-referencing duplicate articles and articles published in languages other than English, studies on animals, social and lifestyle factors, mood disorders, and substance use were excluded. Of these, a total of 122 publications only focusing on smoking and depression in the adolescent population were selected to synthesize this qualitative systemic review. These include 110 original research articles, 8 meta-analysis and reviews, and 4 reports and websites.     

Conclusion
The relationship between smoking and depression in the literature does not reflect the cause-effect relationship. Whereas the evidence on the direction of the association may reflect futile study designs, confounding factors and/or use of indirect measure of depression and quantification of smoking. Future prospective randomized studies should be aimed at elucidating the causal association.