Isotonitazene is a highly potent benzimidazole-derived synthetic opioid increasingly implicated in fatal intoxications. Despite growing forensic detection, controlled in vivo toxicity data remain limited. We determined the median lethal dose (LD50) of isotonitazene in adult male and female Fischer 344 rats using the Dixon up-and-down procedure following single intraperitoneal administration. Mortality and clinical signs were monitored for 14 days. The estimated LD50 was 0.08 mg/kg (95% CI 0.02996-0.182 mg/kg) in males and 0.25 mg/kg (95% CI 0.04841-0.768 mg/kg) in females, indicating greater male sensitivity. Rapid respiratory depression and neuromuscular rigidity occurred within minutes of dosing and preceded lethality. The narrow separation between survival and death demonstrates a steep dose-response relationship and limited safety margin. These findings provide quantitative data for toxicological risk assessment of nitazene opioids and underscore the importance of sex as a biological variable in opioid toxicity studies.
