Dr. Francois Poulin of the University of Quebec at Montreal said the researchers tracked almost 400 adolescents -- 58 percent girls -- ages 12-18, from a large French-speaking school district in Canada.
They were interviewed annually over a seven-year period about their friendships and use of alcohol and drugs.
Among girls, anti-social behavior and early pubertal maturation accelerated opposite-sex friends -- often older males in out-of-school contexts.
Since the legal drinking age is 18 in Canada, younger girls may use older boys to gain access to alcohol.
"Peer relationships are considered to be one of the main risk factors for substance use. However, for boys, the formation of other-sex friendships is not associated with later substance use problems," Poulin, the study author, said in a statement.
"Boys reported receiving higher levels of emotional support from their other-sex friends, whereas girls receive more support from their same-sex friends. It is possible that having other-sex friends is protective for boys because they gain emotional support and are therefore less likely to engage in problem behavior."
The findings are published in the Journal of Research on Adolescence.
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