Inadequate school sanitation and mental health outcomes among adolescent and young adult girls in Uttar Pradesh and Bihar, India

School infrastructure, particularly sex-specific toilet facilities, plays an important role in creating a safe and supportive learning environment for adolescent girls. Although inadequate or lack of school sanitation has been linked to poor educational outcomes, its relationship with girls’ mental health remains underexplored. This study examined the association between school toilet availability and mental health outcomes among adolescent and young adult girls using longitudinal data from two waves of the Understanding the Lives of Adolescents and Young Adults (UDAYA) survey in Uttar Pradesh and Bihar, India. The analysis included a pooled sample of 12,924 girls aged 10–23 years. The regression analysis from pooled OLS, between-effects and mixed-effects models consistently showed that girls attending schools without toilet facilities had significantly higher poor mental health scores after adjustment for sociodemographic characteristics. In contrast, the individual fixed-effects model showed no significant association, suggesting that the observed relationship is driven primarily by differences between girls rather than by within-individual changes in school toilet availability over time. Subgroup analyses further indicated that the association was more pronounced among girls residing in urban areas, those from Uttar Pradesh, and married or early-married girls, particularly during the second survey wave. Sensitivity analyses based on a balanced panel showed a similar pattern, as reported in the main findings. Supplementary findings further demonstrated that access to school toilet facilities was associated with higher odds of continued school enrolment. However, no significant association was observed with subsequent marriage after adjustment for sociodemographic characteristics. Findings suggest that the absence of school toilet facilities functions as a marker of broader educational and social disadvantage rather than a direct within-individual effect of school sanitation. Thus, strengthening school sanitation should be prioritised alongside broader investments in school infrastructure and supportive learning environments to improve girls’ educational participation and psychological well-being.