Does cannabis use in adolescence predict self‐harm or suicide? Results from a Finnish Birth Cohort Study

OBJECTIVE: Longitudinal studies examining the association between adolescent cannabis use and self‐harm are rare, heterogeneous and mixed in their conclusions. We study this association utilizing a large general population‐based sample with prospective data. METHODS: The Northern Finland Birth Cohort 1986 (n = 6582) with linkage to nationwide register data was used to study the association of self‐reported cannabis use at age 15–16 years and self‐harm and suicide death until age 33 (until year 2018), based on register information. Cox regression analysis with Hazard Ratios (HR) and 95% confidence intervals (CI) was used. Psychiatric disorders, parental psychiatric disorders and other substance use were considered as confounders. RESULTS: In all, 6582 (49.2% male) were included in the analysis, and 377 adolescents (5.7%) reported any cannabis use until the age of 15–16 years. Based on register information, 79 (55.7% male) had visited in health care services due to self‐harm, and 22 (90.1% male) had died by suicide. In crude analyses, adolescent cannabis use was associated with self‐harm (HR = 3.93; 95% CI 2.24–6.90). The association between cannabis use and self‐harm remained statistically significant after adjusting for sex, psychiatric disorders at baseline, frequent alcohol intoxications, other illicit drug use, and parental psychiatric disorders (HR 2.06; 95% CI 1.07–3.95). In contrast, the association of cannabis use with suicide did not reach statistical significance even in crude analysis (HR 2.60; 95% CI 0.77–8.78). CONCLUSION: Cannabis use in adolescence may increase risk of self‐harm independent of adolescent psychopathology and other substance use.