II. Indications
- See Sedating Antihistamine
- Local Anesthesia for Lidocaine Allergy
III. Mechanism
- Sedating Antihistamine
- H1 Receptor Antagonist
- Ethanolamine with strong Anticholinergic activity
IV. Dosing: Standard
- Adult (age >=12 years): 25-50 mg PO/IV/IM every 6 hours (maximum: 300 to 400 mg/day)
- Child (age <12 years): 5 mg/kg/day divided four times daily (12.5 mg/5 ml)
- Age 2 to 5 years: 6.25 mg every 4 hours as needed
- Age 6 to 12 years: 12.5 mg every 4 hours as needed
V. Dosing: Local Injection
- Diphenhydramine may be used for local injection when patients are allergic to other Local Anesthesia
- Diphenhydramine 1% (for Local Anesthesia)
- Dilute Diphenhydramine 5% in Normal Saline (1:4)
- Longer delay to onset of action than Lidocaine
- Shorter duration of action than Lidocaine
VI. Adverse Effects
- See Sedating Antihistamine
-
Poisoning
- See Anticholinergic Toxicity
- Mild Overdose
- Causes sedation and muscarinic Anticholinergic Toxicity (Dry Mouth, Tachycardia, Mydriasis, Urinary Retention)
- Severe Overdose
- Causes Agitation, Delirium, Hallucinations, Seizures and coma
- Diphenhydramine blocks myocardial Sodium and Potassium channels (Wide QRS, QTc Prolongation)
- Diphenhydramine Overdose is responsible for >3% of U.S. Overdose deaths (within top 15 drugs)
- References
- Lacey and Dietrich (2023) Crit Dec Emerg Med 37(3): 18-9
VII. Safety
- Pregnancy Category B
- Avoid in Lactation
VIII. Pharmacokinetics
- Duration 4 to 6 hours
- Hepatic metabolism
- Renal Clearance
IX. Resources
- Diphenhydramine film coated tablet (DailyMed)
X. References
- Olson (2020) Clinical Pharmacology, Medmaster Miami, p. 90
- Hamilton (2020) Tarascon Pocket Pharmacopoeia