II. Indications

III. Mechanism

  1. Sedating Antihistamine
  2. H1 Receptor Antagonist
  3. Ethanolamine with strong Anticholinergic activity

IV. 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

V. 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

VI. 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

VII. Safety

  1. Pregnancy Category B
  2. Avoid in Lactation

VIII. Pharmacokinetics

  1. Duration 4 to 6 hours
  2. Hepatic metabolism
  3. Renal Clearance

X. References

  1. Olson (2020) Clinical Pharmacology, Medmaster Miami, p. 90
  2. Hamilton (2020) Tarascon Pocket Pharmacopoeia

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Related Studies

Cost: Medications

diphenhydramine (on 12/21/2022 at Medicaid.Gov Survey of pharmacy drug pricing)
DIPHENHYDRAMINE 12.5 MG/5 ML Generic OTC $0.01 per ml
DIPHENHYDRAMINE 25 MG CAPSULE Generic OTC $0.03 each
DIPHENHYDRAMINE 25 MG TABLET Generic OTC $0.04 each
DIPHENHYDRAMINE 50 MG CAPSULE Generic OTC $0.02 each
DIPHENHYDRAMINE 50 MG/ML VIAL Generic $0.85 per ml
banophen (on 12/21/2022 at Medicaid.Gov Survey of pharmacy drug pricing)
BANOPHEN 25 MG CAPSULE Generic OTC $0.03 each
BANOPHEN 25 MG TABLET Generic OTC $0.04 each
BANOPHEN 50 MG CAPSULE Generic OTC $0.02 each
BANOPHEN ANTI-ITCH 2% CREAM Generic OTC $0.05 per gram
diphen (on 12/21/2022 at Medicaid.Gov Survey of pharmacy drug pricing)
DIPHENHIST 25 MG CAPSULE Generic OTC $0.03 each
DIPHENOXYLATE-ATROPINE 2.5-0.025 MG TABLET Generic $0.18 each
siladryl (on 12/21/2022 at Medicaid.Gov Survey of pharmacy drug pricing)
SILADRYL 12.5 MG/5 ML LIQUID Generic OTC $0.02 per ml