A randomized trial to evaluate the course of effects of a program to prevent adolescent depressive symptoms over 12 months

► Positive intervention effects were found on girls' depressive symptoms. ► No intervention effects were found on boys' depressive symptoms. ► Intervention effects on girls increased to 6-month follow-up. ► Intervention effects on girls remained stable from 6 to 12-month follow-up. ► Total knowledge (cognitive and social) marginally explained the effect.