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Background and Aims: Studies on adolescent alcohol use and cognition are often unable to separate the potential causal effects of alcohol use on cognition from shared etiological influences, including genetic influences or other substance use comorbidities also known to be associated with cognition, such as nicotine use. The present study aimed to fill this gap and clarify the relationship between adolescent alcohol use and young adult cognition by accounting for both measured and unmeasured confounders. Design: A random effects model accounting for nesting in families was used to control for measured confounders. Next, co‐twin comparisons were conducted within the full sample and in monozygotic twin pairs (MZ) to control for unmeasured genetic and environmental confounders shared by co‐twins. Participants/Setting: Participants were 812 individuals (58.6% female, 361 complete pairs, 146 MZ pairs) from the longitudinal FinnTwin12 study in Finland. Measurements Adolescent alcohol use was indexed with measures of frequency of use and intoxication averaged across ages 14 and 17. Cognitive outcomes were measured at average age 22 and included Trail Making Test, California Stroop test, Wechsler Adult Intelligence subtests (Vocabulary, Block Design, Digit Symbol), Digit Span subtest of Wechsler Memory Scale, Mental Rotation Test and Object Location Memory test. Covariates included sex, parental education, general cognitive ability, current alcohol use and nicotine use. Findings Greater frequency of alcohol use and frequency of intoxication across adolescence was associated with decreased vocabulary scores in the co‐twin control [freq: stnd beta = −0.12, 95% confidence interval (CI) = −0.234, −0.013] and MZ only co‐twin control models (freq: stnd beta = −0.305, 95% CI = –0.523, −0.087; intox: stnd beta = −0.301, 95% CI = ‐0.528, −0.074). Conclusions: In Finland, there appears to be little evidence that adolescent alcohol use causes cognitive deficits in young adulthood, except modest evidence for association of higher adolescent alcohol use with lower young adult vocabulary scores. [ABSTRACT FROM AUTHOR]