The purpose of this study was to assess the association between attention-deficit/hyperactivity disorder (ADHD) medication adherence and adverse health outcomes in older adolescents transitioning to adulthood. In a cohort of 17-year-old adolescents with ADHD at Kaiser Permanente Northern California, we assessed medication adherence (medication possession ratio ≥70%) and any medication use and associations with adverse outcomes at 18 and 19 years of age. We conducted bivariate tests of association and multivariable logistic regression models. Adherence declined from 17 to 19 years of age (36.7%-19.1%, p < .001). Non-white race/ethnicity, lower estimated income, and male sex were associated with nonadherence. Model results show nonadherent females experienced several adverse outcomes: Adherence at 18 years of age (referent: nonadherence) was associated with lower odds of pregnancy (adjusted odds ratio [AOR]: .13, 95% confidence interval [CI]: .03-.54). Any use (referent: nonuse) at 18 years of age was associated with lower odds of sexually transmitted infections among females (AOR: .39, 95% CI: .19-.83), pregnancies (AOR: .26, 95% CI: .13-.50), emergency department visits (AOR: .69, 95% CI: .55-.85), and greater odds of injuries (AOR: 1.16, 95% CI: 1.02-1.32). Adherence at 19 years of age was associated with lower odds of pregnancy (AOR: .13, 95% CI: .02-.95). Any use at 19 years of age was associated with lower odds of injury in females (AOR: .77, 95% CI: .60-.99) pregnancy (AOR: .35, 95% CI: .16-.78), and, in both sexes, substance use (AOR: .71, 95% CI: .55-.92). Late adolescence is associated with decline in ADHD medication use and adherence. ADHD medication adherence and any ADHD medication use is associated with fewer adverse health outcomes, particularly in females.