Background: There is an increasing demand for mental health services for young people, which may vary across the year.

Objective: To determine whether there are seasonal patterns in primary care antidepressant prescribing and mental health issues in adolescents and young adults.

Methods: This cohort study used anonymised electronic health records from general practices in England contributing to QResearch. It included 5 081 263 males and females aged 14-18 (adolescents), 19-23 and 24-28 years between 2006 and 2019. The incidence rates per 1000 person-years and the incidence rate ratios (IRRs) were calculated for the first records of a selective serotonin reuptake inhibitor (SSRI) prescription, depression, anxiety and self-harm. The IRRs were adjusted for year, region, deprivation, ethnic group and number of working days.

Findings: There was an increase in SSRI prescribing, depression and anxiety incidence in male and female adolescents in the autumn months (September-November) that was not seen in older age groups. The IRRs for SSRI prescribing for adolescents peaked in November (females: 1.75, 95% CI 1.67 to 1.83, p<0.001; males: 1.72, 95% CI 1.61 to 1.84, p<0.001, vs in January) and for depression (females: 1.29, 95% CI 1.25 to 1.33, p<0.001; males: 1.29, 95% CI 1.23 to 1.35, p<0.001). Anxiety peaked in November for females aged 14-18 years (1.17, 95% CI 1.13 to 1.22, p<0.001) and in September for males (1.19, 95% CI 1.12 to 1.27, p<0.001).

Conclusions: There were higher rates of antidepressant prescribing and consultations for depression and anxiety at the start of the school year among adolescents.

Clinical implications: Support around mental health issues from general practitioners and others should be focused during autumn.