LAUSR

BACKGROUND AND OBJECTIVES Being among the youngest within a school class is linked to disadvantages in various educational and mental health domains. This study aimed to investigate whether preterm born infants are particularly vulnerable to relative age effects on mental health, not previously studied. METHODS We used registry data on all Norwegians born between 1989 and 1998 to compare prescription status for psychostimulants, antidepressants, hypnotics, anxiolytics, and antipsychotics per year from age 10 to 23 years (2004-2016) between exposure groups with different time of birth in the year (relative age) and different gestational age (preterm versus term). RESULTS Of 488 470 individuals, 29 657 (6,1%) were born preterm. For term born in November/December, the adjusted odds ratio (aORs) for psychostimulant prescription compared with peers born in January/February was 1.80 (95% confidence interval [CI], 1.69-1.91) at ages 10 to 14 years, and 1.17 (95% CI, 1.08-1.27) at ages 20 to 23 years. Within preterm born, the corresponding results were 1.39 (95% CI, 1.13-1.69) and 1.34 (95% CI, 1,00-1.78) at ages 10 through 14 and 20 through 23 years, respectively. CONCLUSIONS Being relatively young within the school group was associated with increased psychostimulant prescription in the preterm as well as the term population. In contrast to term peers, the relative age effect for psychostimulant prescription seemed to persist to young adulthood for the preterm population. The results suggest that preterm individuals are vulnerable to long-term effects of relative immaturity and that they require careful consideration from both health care professionals and the school system.