Background <p>Post-pandemic societal contexts and mental health landscape plausibly differ from those in peri-pandemic times. Peri-to-post-pandemic changes in self-injurious thoughts and behaviors (SITB) prevalence may be associated with changes in prevalence of SITB determinants (endowment effect) and their associations with SITB outcomes (coefficient effect). These patterns may vary across at-risk subgroups, particularly bullying perpetrators and victims. However, evidence remains limited. This study investigated how these effects were related to peri-to-post-pandemic changes in SITB prevalence.</p> Methods <p>Using population-representative surveys from peri-pandemic (2021, 2022; <i>N</i> = 2,986) and post-pandemic (2024; <i>N</i> = 1,500) periods, multivariate logistic regressions identified associations between bullying involvement and SITB. We then conducted a threefold non-linear Blinder-Oaxaca decomposition with SITB psychosocial determinants as covariates and differences in suicidal ideation (SI), self-harm (SH), and suicide attempt (SA) prevalence as separate outcome variables. Analyses were independently conducted for the total sample and subsamples stratified by bullying involvement, i.e., perpetrators, victims, and those uninvolved.</p> Results <p>Prevalence of SI, SH, and SA in the young population increased post-pandemic. SI and SH increases were associated with endowment and coefficient effects of depressive and anxiety symptomatology. Bullying involvement was strongly associated with SI, SH, and SA. Perpetrators’ SITB prevalence decreased post-pandemic and was related to determinants’ endowment and coefficient effects. Victims’ SITB prevalence increased, with the increase for SI mainly linked to determinants’ coefficient effect.</p> Conclusions <p>Efforts in safeguarding adolescents and young adults’ mental wellbeing need to be improved and sustained. Tailored strategies to better care for bullying victims need to be explored.</p>