Psychotic disorders are frequently preceded by depressive disorders, and it has been hypothesized that treatment of depression in youth may reduce risk for later psychosis. Using quasi-experimental methods, we estimated the causal relationship between the treatment of adolescent depression with selective serotonin reuptake inhibitors (SSRIs) and the risk of later psychosis.

We used data linkage from multiple national Finnish registries for all individuals (n = 697,289) born between 1987 and 1997 to identify depression diagnosed before age 18, cumulative SSRI treatment within three years of diagnosis, and diagnoses of non-affective psychotic disorders by end of follow-up (age 20–29). We used instrumental variable analyses, exploiting variability in prescribing across hospital districts to estimate causal effects. Analyses were conducted using two-stage least squares modelling. Sensitivity analyses examined effects stratified by confounders and effects of specific SSRIs.

Our final sample included 22,666 individuals diagnosed with depression in adolescence, of whom 60.2% (n = 13,650) had used SSRIs. 10.7% of adolescents with depression went on to be diagnosed with a non-affective psychotic disorder. SSRI treatment for adolescent depression was not associated with a reduced risk of developing a psychotic disorder (one-year β = 0.04,CI:−0.01 to 0.09; two-years β = 0.02,CI:−0.06 to 0.09; three-years β = −0.02,CI:−0.08 to 0.05).

Our quasi-experimental investigation does not support the hypothesis that treatment of adolescent depression reduces the subsequent risk of psychosis. Our findings question the assumption that treatment of common mental health disorders in youth may impact the risk of developing severe mental illnesses in adulthood.