Acute toxicity and sex-dependent lethality of isotonitazene in fischer 344 rats

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 (LD 50 ) 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 LD 50 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.