Diphenhydramine

General

• Type: Antihistamines / H1 blocker
• Dosage Forms: Oral, IM, IV
• Common Trade Names: Benadryl

Adult Dosing

• Allergic reaction
  • Oral: 25-50mg q4-8h, max 300mg/day
  • IM/IV: 10-50mg/dose
• Parkinsonian symptoms
  • Oral: 25-50mg tid-QID
  • IM/IV: 10-50mg/dose
• Insomnia: 50mg PO qhs
• Injectable 1% diphenhydramine (10 mg/mL) can be used as a local anesthestic alternative to ester/amide anesthetics
  • Sodium channel blocker mechanism
  • 1-2 mL of 1% diphenhydramine at a time, to not exceed excessive sedation dose
  • Typical vial is 50 mg/mL, so to make 10 mg/mL:
    • 10 mL NS removed from 50 mL vial
    • Add 10 mL of 50 mg/mL diphenhydramine to 40 mL of NS
    • Sedation is dose related and is similar to what would be expected for IM doses
    • Relative contraindications are the same for IM diphenhydramine administration

Pediatric Dosing

• Allergic reaction
  • Oral/IM/IV: 5mg/kg/day divided q6-8h (1.25-1.67mg/kg/dose)
  • Alternate PO dosing
    • 2-6 yo: 6.25mg q4-6h
    • 6-12 yo: 12-25mg q4-6h
    • >12 yo: Adult dosing

Special Populations

• Pregnancy Rating: B
• Lactation Risk: Enters breast milk
• Renal Dosing: n/a
• Hepatic Dosing: n/a

Contraindications

• Allergy to class/drug

Adverse Reactions

Serious

• Anticholinergic toxicity
• CNS depression
• Cardiac arrthymias (Sodium channel blockade)
• Seizures

Common

• Sedation

Overdose related

• Opsoclonus - conjugate irregular and nonrhythmical jerking movements of the eyes was reported in a 20-year-old woman who had ingested 5 grams

Pharmacology

• Half-life: Children 5 hours, Adults 9 hours, Elderly 13.5 hours
• Metabolism: Hepatic
• Excretion: Urine
• Mechanism of Action: Antihistamines / H1 blocker

See Also

• Antihistamines
• Anticholinergic toxicity