Diphenhydramine (Benadryl) Overdose: 24 Tablets
A 24-tablet diphenhydramine overdose (600 mg total, assuming 25 mg tablets) is a life-threatening emergency requiring immediate emergency department evaluation, with expected anticholinergic toxicity including delirium, seizures, cardiac dysrhythmias with QRS widening, and potential cardiac arrest—treatment centers on supportive care with benzodiazepines for seizures/agitation, sodium bicarbonate for QRS widening >100 ms, and aggressive airway management.
Expected Clinical Presentation
Anticholinergic Syndrome (Dominant Feature)
The patient will likely develop severe anticholinergic toxicity within 1-4 hours of ingestion 1, 2:
- Central nervous system: Delirium, agitation, hallucinations, confusion, bizarre behavior, and potentially status epilepticus (though uncommon, it is distinctly possible) 3, 2
- Cardiovascular: Tachycardia progressing to wide-complex tachycardia with QRS prolongation >100-127 ms due to sodium channel blockade 2, 4
- Neurological: Choreic involuntary movements, muscle rigidity, seizures (generalized or status epilepticus lasting >30 minutes) 1, 2
- Autonomic: Mydriasis, dry mucous membranes, urinary retention, hyperthermia, flushed skin 4
- Respiratory: Potential respiratory depression requiring intubation 5, 2
Severity Assessment
At 600 mg (24 × 25 mg tablets), this exceeds the toxic threshold of 7.5 mg/kg for most adults (approximately 300 mg for patients ≥6 years) 4. Blood concentrations in fatal/near-fatal cases range from 1.26-2.37 µg/mL 1 to 1200 ng/mL 2, with this dose likely producing concentrations in the toxic range.
Critical pitfall: Unlike typical anticholinergic presentations, diphenhydramine overdose uniquely causes sodium channel blockade similar to tricyclic antidepressants, manifesting as QRS widening and ventricular dysrhythmias 6, 2, 4.
Immediate Management Algorithm
1. Airway Protection (First Priority)
- Intubate early if any signs of decreased consciousness, active seizures, or inability to protect airway 2, 4
- Do NOT delay intubation waiting for symptoms to worsen—respiratory failure and aspiration are imminent risks
2. Seizure Management
- Benzodiazepines are first-line: Lorazepam 2-4 mg IV or diazepam 5-10 mg IV 6, 4
- If seizures persist (status epilepticus), escalate to propofol infusion for sedation 1, 2
- Expect that standard benzodiazepine doses may be insufficient—one case required 8 mg lorazepam before propofol controlled seizures 2
3. Cardiac Monitoring and Sodium Channel Blockade Treatment
Obtain immediate 12-lead ECG looking for:
- QRS duration >100 ms
- Terminal rightward axis deviation in lead aVR
- Ventricular dysrhythmias
If QRS >100 ms or wide-complex tachycardia present:
- Administer sodium bicarbonate 1-2 mEq/kg IV bolus (typically 50-100 mEq in adults) immediately 6, 5, 2, 4
- Repeat boluses until QRS narrows to <100 ms
- Target serum pH 7.50-7.55
- Continuous cardiac monitoring is mandatory
Critical contraindication: Do NOT give physostigmine (despite anticholinergic symptoms) when QRS widening is present—it can precipitate asystole 2, 4. Physostigmine should only be considered in hospital settings without cardiac conduction abnormalities 4.
4. Supportive Care
- IV fluid resuscitation for hypotension
- Continuous cardiac monitoring for dysrhythmias
- Foley catheter for urinary retention monitoring
- Cooling measures if hyperthermic
- Avoid activated charcoal if patient has altered mental status or seizure risk (aspiration hazard) 4
5. Observation Period
- Minimum 6-8 hours observation in emergency department even if initially asymptomatic 4
- Symptoms typically manifest within 4 hours but can be delayed up to 6 hours 4
- If symptomatic, ICU admission is warranted for continuous monitoring
Common Pitfalls to Avoid
Mistaking for isolated anticholinergic toxicity: The sodium channel blockade component requires specific treatment with sodium bicarbonate, not just supportive anticholinergic management 2, 4
Using physostigmine inappropriately: While tempting for anticholinergic symptoms, physostigmine is contraindicated with QRS widening or cardiac conduction delays 2, 4
Underestimating seizure severity: Diphenhydramine can cause refractory status epilepticus requiring aggressive management with propofol, not just standard benzodiazepine doses 1, 2
Delaying intubation: Early airway protection prevents aspiration during seizures or altered mental status 2, 4
Inadequate sodium bicarbonate dosing: Aggressive, repeated boluses may be needed—one case required 200 mEq to narrow QRS 2
Prognosis
With aggressive supportive care, complete neurological recovery is expected even after severe toxicity including cardiac arrest with ROSC 5, 2. Most patients become neurologically intact within 15-48 hours after symptom resolution 1, 2.