How should a diphenhydramine (Benadryl) overdose be managed?

Management of Diphenhydramine (Benadryl) Overdose

Diphenhydramine overdose requires immediate emergency department evaluation for any patient with symptoms beyond mild drowsiness, with aggressive supportive care and sodium bicarbonate as the primary antidote for life-threatening cardiotoxicity.

Immediate Triage and Referral Criteria

All patients meeting ANY of the following criteria require immediate emergency department referral 1:

• Dose-based thresholds:

  • Children <6 years: ≥7.5 mg/kg ingested
  • Patients ≥6 years: ≥7.5 mg/kg OR 300 mg (whichever is less)

• Symptom-based criteria (beyond mild drowsiness):

  • Agitation, hallucinations, or behavioral changes
  • Seizures or staring spells
  • Loss of consciousness or altered mental status
  • Abnormal muscle movements
  • Respiratory depression
  • Any cardiovascular instability

• Intent-based criteria:

  • Suicidal intent
  • Intentional abuse
  • Suspected malicious intent (child abuse/neglect)

Transport via ambulance should be arranged based on patient condition and distance to the emergency department 1.

Pathophysiology and Clinical Presentation

Diphenhydramine overdose causes toxicity through three primary mechanisms 2:

1. Anticholinergic effects: Delirium, hallucinations, hyperthermia, dry mucous membranes, urinary retention, mydriasis
2. Antihistaminic effects: Sedation, CNS depression
3. Sodium channel blockade: QRS widening, ventricular dysrhythmias, seizures, cardiovascular collapse (similar to tricyclic antidepressant toxicity)

The sodium channel blockade becomes clinically significant in severe overdoses and is the primary cause of mortality 3, 4, 5.

Emergency Department Management

Primary Antidotal Therapy

Sodium bicarbonate is the first-line antidote for life-threatening cardiotoxicity from diphenhydramine overdose 3, 4, 5:

• Indication: QRS widening >100 msec, ventricular dysrhythmias, hypotension, or cardiovascular instability
• Dosing:
  • Adults: 1-2 mEq/kg IV bolus (typically 50-100 mEq from 1 mEq/mL solution)
  • Children: 1-2 mEq/kg IV bolus (from 0.5 mEq/mL solution)
• Repeat boluses every 3-5 minutes until QRS narrows
• Target: QRS <100 msec, serum pH 7.50-7.55
• Continuous infusion: May be needed for persistent toxicity (150 mEq in 1L D5W at 1.5-2× maintenance rate)

The mechanism involves both increased extracellular sodium concentration (overcoming sodium channel blockade) and alkalemia (reducing drug binding to sodium channels) 3.

Seizure Management

Benzodiazepines are first-line for seizure control 1, 5:

• Lorazepam: 0.1 mg/kg IV (max 4 mg per dose) OR
• Midazolam: 0.2 mg/kg IM/IV (max 10 mg per dose)
• Repeat dosing as needed for ongoing seizures
• Status epilepticus: May require propofol infusion if benzodiazepines fail 5

Critical caveat: Physostigmine is contraindicated in the presence of QRS widening or ventricular dysrhythmias, as it can precipitate asystole 1, 5. While physostigmine can reverse anticholinergic symptoms, it should only be considered in isolated anticholinergic toxicity without cardiac conduction abnormalities.

Alternative Antidotal Therapy

Rivastigmine transdermal patch may be considered when physostigmine is unavailable 6:

• Dose: 9.5 mg/24-hour patch applied transdermally
• Indication: Severe anticholinergic delirium without cardiac conduction abnormalities
• Evidence is limited to case reports but shows promise as an alternative acetylcholinesterase inhibitor

Intravenous lipid emulsion therapy may be considered for refractory cases 7:

• 20% lipid emulsion: 1.5 mL/kg IV bolus, followed by 0.25 mL/kg/min infusion
• Reserved for severe toxicity unresponsive to sodium bicarbonate
• Limited evidence but has shown benefit in case reports of massive overdose

Supportive Care

Airway management 1, 4:

• Early intubation for respiratory failure, inability to protect airway, or refractory seizures
• Avoid succinylcholine if possible (though not contraindicated like in organophosphate poisoning)

Cardiovascular support:

• Aggressive IV fluid resuscitation for hypotension
• Continuous cardiac monitoring for dysrhythmias
• Avoid Class Ia and Ic antiarrhythmics (will worsen sodium channel blockade)

Decontamination:

• Do NOT induce emesis (risk of rapid deterioration with seizures/loss of consciousness) 1
• Activated charcoal is not recommended in the prehospital setting due to aspiration risk 1
• May consider activated charcoal (1 g/kg, max 50 g) in the ED if patient presents within 1 hour and can protect airway

Observation and Disposition

Home observation is appropriate ONLY if ALL of the following are met 1:

• Dose below toxic threshold (<7.5 mg/kg and <300 mg for patients ≥6 years)
• No symptoms or only mild drowsiness
• No suicidal intent or concerning circumstances
• Reliable caregiver present

• 4 hours post-ingestion with no symptom development

Follow-up call at 4 hours post-ingestion is recommended for home observation cases 1.

Hospital admission criteria:

• Any moderate-to-severe symptoms
• QRS widening or QTc prolongation (>480 msec) 8
• History of cardiac abnormalities or congenital long QT syndrome 8
• Seizures or altered mental status
• Requirement for antidotal therapy

Critical Pitfalls to Avoid

1. Underestimating cardiac toxicity: Normal initial ECG does not exclude subsequent development of conduction abnormalities. Serial ECGs are essential 8, 9.

2. Inappropriate use of physostigmine: Never administer with QRS widening, as this can cause fatal bradycardia or asystole 1, 5.

3. Delayed recognition of sodium channel blockade: Look for QRS widening and terminal rightward axis deviation in lead aVR, similar to TCA toxicity 3, 4.

4. Premature discharge: Symptoms can develop up to 4 hours post-ingestion; observation period must be adequate 1.

5. Missing underlying cardiac conditions: QTc prolongation may unmask latent long QT syndrome, requiring cardiology follow-up 8.

6. Inadequate seizure control: Aggressive benzodiazepine dosing is required; do not hesitate to escalate to propofol for status epilepticus 5.

Special Considerations

Dermal exposures: Thorough skin decontamination with soap and water; 8-hour observation period if asymptomatic 1.

Pediatric patients: Lower threshold for ED referral (7.5 mg/kg) due to increased sensitivity to anticholinergic effects 1.

Cardiac monitoring: Continuous telemetry for minimum 6-8 hours in symptomatic patients; consider advanced imaging (echocardiography with strain analysis) if concern for myocardial dysfunction 9.