['Peer substance involvement (PSI) is a robust correlate of adolescent substance use. A small number of genetically informative studies suggest that shared genetic and environmental factors contribute to this association. We examine mechanisms by which PSI influences the etiology of regular substance involvement (RSI), particularly in women.', 'Population-based cohort study of twin women from the US Midwest.', '2176 twin women.', 'To examine the relationship between self-reported PSI during adolescence and a composite RSI representing regular tobacco, alcohol and cannabis use during young adulthood, using genetically informative correlation, moderation and joint correlation-moderation models.', 'There was evidence for a significant additive genetic X environment interaction. PSI was moderately heritable (h(2) = 0.25). Genetic, shared and non-shared influences on RSI overlapped with influences on PSI (genetic correlation of 0.43). Even after controlling for these shared genetic influences, RSI was more heritable in those reporting greater PSI.'] Does peer substance involvement modify genetic influences on regular substance involvement in young women?