Parental unemployment is associated with worse adolescent mental health, but prior evidence has primarily been based on cross-sectional studies subject to reverse causality and confounding. We assessed the association between parental unemployment and changes in adolescent psychotropic medication purchases, with longitudinal individual-level fixed-effects models that controlled for time-invariant confounding. We used data from a large, register-based panel of Finnish adolescents aged 13-20 years in 1987-2012 (n = 138,644) that included annual measurements of mothers' and fathers' employment and offspring psychotropic medication purchases. We assessed changes in the probability of adolescent psychotropic medication purchases in the years before, during, and after the first episode of parental unemployment. There was no association between mother's unemployment and offspring psychotropic purchases in the fixed-effects models, suggesting this association is largely driven by unmeasured confounding and selection. By contrast, father's unemployment led to a significant 15%-20% increase in the probability of purchasing psychotropic medication among adolescents even after extensive controls for observed and unobserved confounding. This change takes at least 1 year to emerge, but it is long-lasting; thus, policies are needed that mitigate the harm of father's unemployment on offspring's mental well-being.