Same-Sex Friendship, School Gender Composition, and Substance Use: A Social Network Study of 50 European Schools.

Other-sex friendship was associated with smoking, binge drinking, and cannabis use for girls and with smoking for boys. Substance use was more frequent in schools with a majority of males. Conclusions/Importance: Adolescent girls are best protected from substance use if they are in gender-balanced schools, but in same-sex friendship. This offers new perspectives on gender mixing at school. In schools with a majority of boys, more attention should be paid to girls, and gender-specific health promotion programs should be implemented. This European study is the first to take into account both individual (other-sex friendship) and contextual (gender composition of schools) gender interactions. It confirms previous studies on other-sex friendship, while shedding light on the influence of gender-normative contexts on substance use.