RESUMO
Non-suicidal self-injury (NSSI) among adolescents is a major health concern globally. The epidemiology and determinant of NSSI have not been studied extensively in Iran. Thus, this longitudinal study sought to investigate the prevalence, incidence and contributing factors of NSSI among adolescents in Iran. This study was conducted at high school students aged 14-18 years old in the metropolitan city of Tabriz, located in north western Iran. Data were collected from 6229 10th-grade students at two time-points, with 12 months interval in October and November 2017 and 2018. A self-administered questionnaire was used to collect information on demographic, NSSI and cigarette and hookah smoking from the study participants. The results showed that lifetime prevalence of NSSI among students was 6.2% (95% confidence interval [CI]: 5.7 to 6.9) at the first assessment. Scratching and hair pulling were the most and least common methods of self-injury, respectively. Over the one year, 206 students (3.6% CI 95%: 3.1-4.1) reported the onset of NSSI. Tobacco smoking and previous year average grade were related to the incidence of NSSI. Although gender was significantly associated with the prevalence of NSSI, the relationship between gender and incidence of NSSI was not significant. Although the prevalence of NSSI was low among adolescents, the incidence rate was considerable. Further studies are required to understand the risk factors of NSSI in Iran.