Background Adolescence represents a key period in both physical and psychological development. Mental health difficulties in this age group are associated with significant potential harms including lower educational achievement, alcohol and substance misuse, and suicide and self-injury, amongst others. Adolescents with a longstanding illness may face difficulties such as hospital appointments and disruption to routine and education not faced by their healthy peers. Previous longtidinal research has shown associations between individual longstanding illnesses and mental health difficulties outside of the UK, whilst studies in the UK have shown similar relationships, limited to cross-sectional designs in individual conditions. This study therefore explored the relationship between having any longstanding illness and serious psychological distress, or having a previous diagnosis of depression or serious anxiety, at age 17 years, using UK longitudinal data.

Methods A secondary analysis of the UK Millennium Cohort Study was conducted. Outcome measures were dichotomised responses from the self-administered Kessler-6 questionnaire for psychological distress, and self-reported previous diagnosis of depression or serious anxiety. Longstanding illness data and covariates were taken from questionnaires with parents and adolescents, from birth up to age 17 years. Data were analysed using multiple binary logistic regression.

Results 9763 cohort members were included in the final analysis, at an attrition rate of 49.9%. From this sample, 15.9% of cohort members scored above the threshold for serious psychological distress on the Kessler-6, with 18.6% of participants who reported a longstanding illness before age 17 years scoring above the cutoff, compared with 14.2% in the unexposed group. This corresponded to adolescents with a longstanding illness being at an increased odds of both serious psychological distress (odds ratio = 1.44, 95%CI = 1.28-1.60) and having been previously told by a doctor that they have depression or serious anxiety (odds ratio = 1.88, 95%CI 1.64-2.15).

Conclusion Children and young people with longstanding physical illness are at increased risk of having serious psychological distress or anxiety or depression. Practitioners should be aware of this and routine screening for psychological distress, plus additional support, may beneficial.