Diphenhydramine as a Protective Agent in a Rat Model of Acute, Lethal Organophosphate Poisoning

Objective: To evaluate the effects of diphenhydramine chloride (DPH) on mortality in a rat model of acute, severe organophosphate poisoning (OP). Methods: Wistar rats (n = 40) were randomized to pretreatment with either normal saline (controls), 5 mg/kg atropine, 3 mg/kg DPH, 15 mg/kg DPH, or 30 mg/kg DPH given as a single intramuscular injection 5 minutes prior to a subcutaneous injection of 25 mg/kg dichlorvos (n = 8 per group). The primary endpoint was 10‐minute survival. Survival at 24 hours was a secondary endpoint. Comparison of survival rates between groups was carried out by ANOVA and the Student‐Newman‐Keuls test. Results: Dichlorvos exposure resulted in profound fasciculations within 2 minutes of injection in all cohorts. In controls, fasciculations were followed by respiratory arrest within 10 minutes (0% survival). The rats receiving atropine pre‐treatment exhibited similar fasciculations (nicotinic effects) without subsequent respiratory arrest, resulting in a significant improvement in survival (88%, p < 0.001). The DPH‐treated rats exhibited a significant dose‐dependent reduction in mortality, with the 3 mg/kg, 15 mg/kg, and 30 mg/kg groups demonstrating 0%, 25%, and 100% survival, respectively. There was no additional mortality between 10 minutes and 24 hours in any group. There was no significant difference in survival between the high‐dose DPH and the atropine groups. Conclusions: Diphenhydramine chloride significantly reduced mortality in rats with acute, severe dichlorvos exposure.