Drug Summary

Diphenhydramine Hydrochloride (Benadryl®)

Pharmacologic properties:

Diphenhydramine is a histamine H1-receptor antagonist that prevents the release of histamine from effector mast cells. Histamine is a vasoactive substance central to allergic reactions that induces vasodilation, vascular permeability, and bronchoconstriction. Diphenhydramine prevents histamine-mediated responses, particularly the effects of histamine on the smooth muscle of the bronchial airways, skin, gastrointestinal tract, and blood vessels.


  • Acute allergic reactions (mild, moderate, or severe)
  • Anaphylaxis
  • Acute dystonic reactions associated with ingestion of phenothiazines and related drugs (haloperidol, thorazine, compazine, metaclopromide, ziprasidone)


  • Benadryl is not to be used in newborn or premature infants or in nursing mothers
  • Known hypersensitivity to diphenhydramine or antihistamines


  • In premature babies and infants, diphenhydramine in over-dosage may cause convulsions or death
  • May cause significant sedation or paradoxical excitation/akethesia
  • Diphenhydramine has additive effects with alcohol and other CNS depressants
  • Antihistamines may cause dizziness, confusion, delirium, hallucinations, and/or hypotension in the elderly (60 years or older)
  • Diphenhydramine has an atropine-like action and therefore should be used with caution in patients with a history of bronchial asthma, increased intraocular pressure, hyperthyroidism, cardiovascular disease or hyper-tension

Adverse Reactions:

  • Drowsiness, sedation
  • Confusion
  • Vertigo
  • Hyperactivity in children
  • Palpitations
  • Tachycardiac
  • PVC’s
  • Hypotension
  • Nausea
  • Vomiting
  • Diarrhea
  • Dry mouth
  • Constipation
  • Urinary retention
  • Thickening of bronchial secretion
  • Wheezing

Dosage and administration:

  • Adult
    • 1 mg/kg IV or IM (maximum dose 50 mg)
  • Pediatric
    • 1 mg/kg IV or IM (maximum dose 50 mg)