Incident health outcomes among individuals with a sibling diagnosed with a psychiatric disorder

Abstract Background Major psychiatric disorders (MPDs) are associated with substantial burden not only on the affected individuals but also their families. However, little is known about the development of mental and somatic health in sibling relationships. We examined whether exposure to a sibling's MPD is associated with incident mental disorders and somatic conditions among their co-siblings. Methods This register-based cohort study included full siblings of individuals with MPDs diagnosed at age 5-25 in Finland. We defined the reference group as full siblings of individuals without an MPD diagnosis. Data were obtained from national registers. Hazard ratios (HRs) for incident depression, anxiety, antidepressant use, infections, injuries, and metabolic diseases in relation to exposure to a sibling's MPD were calculated using Cox regression. We also modelled time-dependent HRs with four time periods after the date of exposure to a sibling's MPD diagnosis (<3 years, 3-6 years, 6-9 years, >9 years). Results The study sample included 13,101 full siblings of individuals with MPDs and 77,739 unexposed comparators. Individuals exposed to a sibling's MPD experienced consistently higher rates of adverse mental health outcomes, especially within the first 3 years after the exposure (e.g., HR for depression: 1.98, 95% CI: 1.75-2.24). Exposure to a sibling's MPD was also associated with the incidence of injuries, infections, and metabolic disease (1.09, 95% CI: 1.06-1.12; 1.10, 95% CI: 1.07-1.13; 1.16, 95% CI: 1.06-1.27). Conclusions Exposure to a sibling's MPD was consistently associated with a higher rate of incident depression, anxiety, and antidepressant use. Screening the mental health of siblings of young people with MPDs may help identify high-risk individuals early. Key messages • Exposure to a sibling’s psychiatric disorder is linked to a higher subsequent risk of depression and anxiety in co-siblings. • Screening the mental health of siblings of young people with MPDs may help identify high-risk individuals early.