Pharm

Diphenhydramine

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Diphenhydramine, Benadryl, Banophen, Aller-Max, Diphen, Diphenhist, Dytan, Siladryl, Sominex

  • See Also
  1. Sedating Antihistamine
  • Indications
  1. See Sedating Antihistamine
  2. Local Anesthesia for Lidocaine Allergy
  • Mechanism
  1. Sedating Antihistamine
  2. H1 Receptor Antagonist
  3. Ethanolamine with strong Anticholinergic activity
  • Dosing
  • Standard
  1. Adult (age >=12 years): 25-50 mg PO/IV/IM every 6 hours (maximum: 300 to 400 mg/day)
  2. Child (age <12 years): 5 mg/kg/day divided four times daily (12.5 mg/5 ml)
    1. Age 2 to 5 years: 6.25 mg every 4 hours as needed
    2. Age 6 to 12 years: 12.5 mg every 4 years as needed
  • Dosing
  • Local Injection
  1. Diphenhydramine may be used for local injection when patients are allergic to other Local Anesthesia
  2. Diphenhydramine 1% (for Local Anesthesia)
    1. Dilute Diphenhydramine 5% in Normal Saline (1:4)
    2. Longer delay to onset of action than Lidocaine
    3. Shorter duration of action than Lidocaine
  • Adverse Effects
  1. See Sedating Antihistamine
  2. Poisoning
    1. See Anticholinergic Toxicity
    2. Mild Overdose
      1. Causes sedation and muscarinic Anticholinergic Toxicity (Dry Mouth, Tachycardia, Mydriasis, Urinary Retention)
    3. Severe Overdose
      1. Causes Agitation, Delirium, Hallucinations, Seizures and coma
      2. Diphenhydramine blocks myocardial Sodium and Potassium channels (Wide QRS, QTc Prolongation)
      3. Diphenhydramine Overdose is responsible for >3% of U.S. Overdose deaths (within top 15 drugs)
    4. References
      1. Lacey and Dietrich (2023) Crit Dec Emerg Med 37(3): 18-9
  • Safety
  1. Pregnancy Category B
  2. Avoid in Lactation
  • Pharmacokinetics
  1. Duration 4 to 6 hours
  2. Hepatic metabolism
  3. Renal Clearance
  • Resources
  1. Diphenhydramine film coated tablet (DailyMed)
    1. https://dailymed.nlm.nih.gov/dailymed/drugInfo.cfm?setid=702f9786-7ce9-43e4-921d-e1db09612127
  • References
  1. Olson (2020) Clinical Pharmacology, Medmaster Miami, p. 90
  2. Hamilton (2020) Tarascon Pocket Pharmacopoeia