Diphenhydramine

Aliases: Benadryl  

Indications: Allergic Reaction, Anaphylaxis, Adult dystonic reaction, or Persistent nausea/vomiting    

Contraindications: Known Allergy  

Concentrations: 

Injection: 50 mg/mL

Tablet: 25 mg

Dosing

Adult:

Indication Dose Route Note 
Moderate/Severe Allergic
Reaction or Dystonia		50mgIV/IM/POSlow IV Push
Mild Allergic Reaction
with only hives/rash		25mgIV/IM/POSlow IV Push
Persistent Nausea/Vomiting25mgIV/IMSlow IV Push
May repeat x 1
after 20 minutes

Pediatric:

Indication Dose Route Note 
Mild to Severe Allergic
Reaction or Dystonia		1mg/kg
Max: 50mg		IV/IM/PODo not administer
if < 5kg
Persistent Nausea/Vomiting1mg/kg
Max: 25mg		IV/IMDo not administer
if < 5kg
Use Handtevy or Approved Pediatric Reference Guide for Amount to Administer 

Precautions: 

Antihistamine toxicity: 

  • Remember: “red as a beet, dry as a bone, hot as a hare, blind as a bat, mad as a hatter, and full as a flask.” 
  • Brugada-like ECG patterns are seen with anticholinergic toxicity. 
  • Elimination mechanism concerns 
  • Potent anticholinergic agent 
  • Pregnancy Category B 

Adverse/Side Effects: Mydriasis, photophobia, ataxia, tachycardia, dizziness, drowsiness  

Class: Antihistamine, Ethanolamine, Anticholinergic 

Mechanism of Action: Diphenhydramine blocks the effects of Histamine (H1 histamine) on the H1 receptor site through a competitive competition for the peripheral H1 site. When diphenhydramine is bound the H1 site cannot be stimulated preventing the effects of histamines (swelling, etc.). As an antihistamine, diphenhydramine is one of the most effective antihistamines. 

Onset of ActionPeak EffectDuration of Action 
Rapid (Injection)
Varies (PO)		1-3 hours6-12 hours