Introduction:
Research suggests that smoking is positively associated with depressive symptoms at different stages of life. However, the prospective association between smoking and depressive symptoms as well as the direction of this relationship in the transition from adolescence to adulthood remains unclear. Consequently, this study aimed to analyse the prospective reciprocal associations between smoking and depressive symptoms across this transitional period.

Material and Methods:
A Norwegian population-based sample of 2936 participants (54.16% women) were assessed in 1992 (T1; early-mid adolescence; Mage = 14.84 years; cigarettes per day [M = 10.49, SD = 6.44]), 1994 (T2; late adolescence); 1999 (T3; emerging adulthood); 2005 (T4; adulthood); and 2020 (T5; mid adulthood). Responses were analysed using a prospective random intercept cross-lagged panel model (RI-CLPM). Smoking frequency and depressive symptoms (Hopkin Symptoms Checklist, 6 items) were assessed. Multigroup analyses were run by gender and socioeconomic status.

Results:
The RI-CLPM overall model showed an adequate fit (Comparative Fit Index [CFI] = 0.99, Tucker-Lewis Index [TLI] = 0.98, and Root Mean Square Error of Approximation [RMSEA] = 0.03). Greater depression in T1 predicted higher smoking status in T2 (β = 0.10, p < .001). Conversely, adolescent smoking at T1 predicted later adolescent depressive symptoms at T2 (β = 0.09, p < .001). No other significant cross-lagged effects emerged. Satorra-Bentler Scaled Chi-square tests showed that the model did not differ by gender (Δχ2(21) = 26.39, p = .192) nor by SES (Δχ2(21) = 29.05, p = .113).

Conclusions:
The present study provides preliminary evidence for a reciprocal pattern in the relationship between cigarette smoking and depressive symptoms throughout adolescence. Cigarette smoking prevention efforts should focus on both depressive symptoms and cigarette smoking during the whole period of adolescence.