2024 Grade‐Specific Prevalence and Adjacent‐Grade Differences in Illicit Drug Use Among Japanese Junior and Senior High School Students

Abstract Aim Effective school‐based prevention of illicit drug use among adolescents requires empirical guidance on when to intervene. We aimed to estimate grade‐specific prevalence and adjacent‐grade differences (grades 7–12) in illicit drug use and test whether co‐occurring behaviors and psychosocial factors modified grade patterns. Methods We analyzed two concurrent national school‐based surveys in 2024 using a stratified cluster design with design‐based weights. The sample comprised 88,356 students. Outcomes were past‐year and lifetime illicit drug‐use prevalence. Survey‐weighted logistic regression yielded grade‐specific prevalence and adjacent‐grade contrasts controlled by Benjamini–Hochberg false discovery rate (FDR). Grade‐by‐factor interactions were tested for past‐year alcohol use, tobacco use, medicine misuse, perceived access to illicit drugs, temptation to use, permissive attitudes, and refusal self‐efficacy. Results National estimates showed a past‐year prevalence of 0.44% (95% CI, 0.37–0.52) and a lifetime prevalence of 0.88%. Prevalence increased with grade showing low levels in grades 7–9 (0.12%, 0.12%, 0.05%) and higher levels in grades 10–12 (0.73%, 0.88%, 0.89%); lifetime estimates showed similar patterns (0.19%–0.22% in grades 7–8 vs. 1.39%, 1.62%, 2.33% in grades 10–12). The grade 9 → 10 contrast remained significant after FDR adjustment (past year: +0.71 percentage points; OR, 12.63). FDR‐adjusted interactions were significant for alcohol and tobacco use, permissive attitudes, perceived access, and refusal self‐efficacy. Conclusions Although its overall prevalence was low, illicit drug use increased with grade, particularly at the grade 9–10 transition. Prevention programs could target this transition by addressing alcohol/tobacco use, permissive attitudes, perceived accessibility, and refusal self‐efficacy.