Deliberate self-poisoning (DSP) is the most common suicide method and can be life-threatening. The purpose of this study was to investigate the factors related to the lethality of DSP and the characteristics of the adolescent group.A retrospective study was conducted on patients who had visited an academic hospital's regional emergency medical center between 2015 and 2018. The data reviewed through their medical records included sociodemographic factors, clinical variables, and psychiatric treatment. Four groups (Q1-Q4) were categorized by descriptive analysis using the risk-rescue rating scale.A total of 491 patients were enrolled in this study. This study showed that high lethality had statistically significant associa-tions with male sex, older age, admitting suicidal intentions, and the use of herbicides for suicide. Logistic regression analyses showed a significant association between high-lethality and female [odds ratio (OR)=0.50, 95% confidence interval (CI)=0.30-0.81, p=0.01], non-psychiatric drugs (over-the-counter drug: OR=2.49, 95% CI=1.08-5.74, p=0.03; herbicide: OR=8.65, 95% CI=3.91-19.13, p<0.01), and denial of suicide intent (OR=0.28, 95% CI=0.15-0.55, p<0.01).This study showed the clinical factors associated with the high lethality of DSP and suggested that efforts were needed to care for and thoroughly examine patients with DSP.