The findings of this prospective analysis suggest that early tobacco use, particularly initiation of regular use before age 14, is longitudinally associated with SRB at age 22, especially among females. Namely, daily cigarette smoking at age 14 was associated with intentional self-injury, suggesting that daily smoking adolescents are at higher risk for self-injurious behaviors, such as cutting or burning oneself, compared to never users. Exploring early smoking in a young population sample extends earlier research during an important period of risk for developing these behaviors and may be an opportune time for preventive actions. Results of pairwise comparisons of exposure- and outcome-discordant twin pairs suggest that the associations found in individual-level analyses are not due to complete confounding by familial factors. However, as these within-pair estimates did not reach statistical significance, their interpretation remains speculative. Also, limited samples of discordant twin pairs constrain power and those results are suggestive but not definitive; we cannot confidently dismiss potential confounding.
