What is the management approach for a diphenhydramine (Benadryl) overdose?

Management of Diphenhydramine (Benadryl) Overdose

Diphenhydramine overdose requires immediate medical intervention with focus on supportive care, cardiac monitoring, and management of anticholinergic toxicity to prevent mortality and serious morbidity.

Clinical Presentation

Diphenhydramine overdose presents with a spectrum of symptoms due to its anticholinergic properties:

Mild to Moderate Toxicity

• Anticholinergic effects: dry mouth, blurred vision, urinary retention, tachycardia
• CNS effects: drowsiness, confusion, agitation
• Cardiovascular: mild tachycardia, hypertension

Severe Toxicity

• Neurological: hallucinations, delirium, seizures, status epilepticus
• Cardiovascular: wide-complex tachycardia, QRS prolongation, hypotension
• Hyperthermia
• Rhabdomyolysis
• Coma

Management Algorithm

1. Initial Stabilization

• Secure airway, breathing, circulation (ABCs)
• Establish IV access
• Continuous cardiac monitoring
• Pulse oximetry
• Obtain baseline ECG to assess for QRS widening

2. Decontamination

• For recent ingestions (<1 hour) with no contraindications:
  • Consider activated charcoal 1g/kg (if patient is alert or airway is protected)
  • Avoid gastric lavage unless massive recent ingestion with protected airway 1

3. Cardiovascular Management

• For wide-complex tachycardia or QRS >100 ms:

  • Administer sodium bicarbonate 1-2 mEq/kg IV bolus
  • Consider continuous sodium bicarbonate infusion for persistent QRS widening
  • Target serum pH 7.45-7.55 1

• For hypotension:

  • IV fluid resuscitation with normal saline (1-2 L in adults)
  • If refractory to fluids, consider vasopressors:
    • Dopamine (400 mg in 500 mL of 5% dextrose) at 2-20 μg/kg/min 2
    • Norepinephrine if dopamine fails

4. Neurological Management

• For seizures:

  • Benzodiazepines first-line: Lorazepam 2-4 mg IV (adults) or 0.05-0.1 mg/kg (children)
  • If seizures persist, consider phenobarbital or propofol 2
  • Intubation and mechanical ventilation may be necessary for status epilepticus

• For agitation/delirium:

  • Benzodiazepines preferred over antipsychotics
  • Lorazepam 1-2 mg IV/IM or midazolam 2-5 mg IV/IM 2

5. Anticholinergic Toxicity Management

• Physostigmine consideration:
  • Reserved for severe cases with life-threatening anticholinergic delirium, hyperthermia, or tachyarrhythmias
  • Contraindicated if QRS prolongation or heart block present
  • Dosing: 1-2 mg IV slowly (adults), 0.02 mg/kg (children) 3
  • Must have atropine immediately available as antidote

6. Supportive Care

• Temperature management for hyperthermia
• Cooling measures if temperature >39°C
• IV fluids for rhabdomyolysis
• Consider benzodiazepines for severe agitation

7. Monitoring and Disposition

• Minimum observation period of 6 hours for mild cases
• Severe cases require ICU admission with continuous cardiac monitoring
• Monitor for:
  • Cardiac arrhythmias
  • Seizures
  • Rhabdomyolysis (check CK levels)
  • Renal function

Special Considerations

Massive Overdose

In cases of massive overdose (>1 gram in adults):

• Early intubation may be necessary for airway protection
• More aggressive sodium bicarbonate therapy for cardiotoxicity
• Consider extracorporeal removal techniques in extreme cases 4

Mixed Overdoses

• Common with other medications (especially NSAIDs, acetaminophen)
• Obtain appropriate drug levels
• Consider specific antidotes for co-ingestants (e.g., NAC for acetaminophen) 2

Pitfalls to Avoid

1. Failure to recognize cardiac toxicity - QRS widening may precede cardiovascular collapse
2. Delayed treatment of seizures - Can progress to status epilepticus
3. Overreliance on physostigmine - Use cautiously and only in specific circumstances
4. Inadequate monitoring - Even seemingly stable patients can deteriorate rapidly
5. Premature discharge - Patients require adequate observation period due to risk of delayed toxicity

Laboratory Evaluation

• Complete blood count
• Comprehensive metabolic panel
• Creatine kinase
• Urinalysis (for myoglobinuria)
• ECG (serial monitoring)
• Consider toxicology screen for co-ingestants

Remember that diphenhydramine overdose can be life-threatening, with death reported in cases of massive ingestion (>7.5 g) 4. Early recognition and aggressive management of cardiovascular and neurological complications are essential for improving outcomes.