Abstract
Adolescence and early adulthood represent periods of heightened vulnerability to mental health disorders. In Norway, mental health care shifts from child and adolescent mental health services (CAMHS) to adult mental health services (AMHS) at age 18. This transition is often fragmented, with risks of treatment discontinuity, disengagement, and increased reliance on general practitioners (GPs). The aim of this study was to examine health service utilization patterns between ages 15.5 and 21, covering the critical transition from CAMHS to AMHS. We conducted a population-wide longitudinal registry study including all Norwegians aged 15.5–21 between 2008 and 2021 (N = 1,313,077). Data were obtained from the Norwegian Patient Registry, the Control and Payment of Health Reimbursement (KUHR) database, and Statistics Norway. We examined specialist mental health, GP, out-of-hours GP, and somatic service use. Subgroup analyses included individuals with a P-diagnosis recorded in GP records (N = 372,108) and those with CAMHS contact at age 17 (N = 71,779). Service utilization was modeled using Poisson regression with clustered standard errors and adjustment for sex and calendar year. Specialist mental health service use peaked at age 16, declined sharply after age 18 and reached its lowest levels around age 20. In contrast, GP and out-of-hours GP consultations increased steadily through late adolescence, particularly after age 18. These patterns were consistent in subgroup analyses of adolescents with a P-diagnosis and those with recent CAMHS contact. Somatic and acute somatic health care use remained stable.