BACKGROUND: Parental death and its anniversaries, including anticipation of these dates, might cause distress and increase the risk of substance use disorder and suicide-related behaviour in bereaved adolescents and young adults. We examined whether the risk of substance use disorder and suicide-related behaviour increases around the date of parental death and subsequent anniversaries.

METHODS: Using Swedish national registers, we conducted a cohort study of individuals aged 12-24 years. We included individuals aged 12-24 years between Jan 1, 2001, and Dec 31, 2014, whose parents were alive at entry (n=1 858 327) and followed up with them until the end of age 24 years. We excluded individuals with a half-sibling, a history of emigration, a previous record of the outcome events, a parental death before study entry, two parental deaths on the same day during the follow-up, or missing data for relevant variables. Follow-up ended on the day of an outcome event or on Dec 31, 2014; at age 25 years, emigration, or death; or a year before the second parental death. We studied substance use disorder and suicide-related behaviour outcomes separately and included non-fatal and fatal events in both outcomes. We used Cox regression to estimate hazard ratios (HRs), controlling for baseline psychiatric, demographic, and socioeconomic characteristics. Parental death was modelled as a time-varying exposure over 72 monthly periods, starting from 1 year before the parental death to the fifth year and later after the death. Unmeasured confounding was also addressed in within-individual comparisons using a case-crossover design.

FINDINGS: During follow-up, there were 42 854 substance use disorder events, with a crude rate of 3·1 per 1000 person-years. For suicide-related behaviour, there were 19 827 events, with a crude rate of 1·4 per 1000 person-years. Most of the events studied were non-fatal. In the month of parental death, the HR for substance use disorder risk was 1·89 among male participants, and, for suicide-related behaviour, was 3·76 among male participants and 2·90 among female participants. In male participants, there was an increased risk around the first anniversary (substance use disorder: HR 2·64 [95% CI 1·56-4·46] during the anniversary month; 2·21 [1·25-3·89] for the subsequent month; and for suicide-related behaviour: 3·18 [1·32-7·66] for the subsequent month). Among female participants, an increased risk of substance use disorder recurred around every year consistently in the month before the anniversary of the death and there was an increased risk for suicide-related behaviour in the months of the first and second anniversaries.

INTERPRETATION: Although effect sizes were large in this cohort study, the number of individuals who had the outcomes was small. Nevertheless, adolescents and young adults, especially women and girls, who had the death of a parent showed increased risk of substance use disorder and suicide-related behaviour around the first few death anniversaries. Adolescents and young adults, especially women and girls, who had the death of a parent could benefit from preventive measures to reduce distress around the first few years of death anniversaries.