Researchers took a group of teenagers who had never smoked and for one year tracked whether they took up smoking and how their mental health changed  Those who began smoking within a month of the study' s start were four times as likely to be depressed at the study' s end as those who did not begin smoking
Since nicotine in cigarettes changes brain chemistry, perhaps thereby affecting mood, it is likely that smoking contributes to depression in teenagers
Participants who were depressed at the study's start were no more likely to be smokers at the study's end than those who were not depressed.