Diphenhydramine*

• Diphenhydramine

• Benadryl

Classification:

• Antihistamine, Anticholinergic

Mechanism of Action:

• Competes with free histamine for binding and blocks H1 histamine receptors
• Antagonizes the effects of histamine on Histamine (HA) receptors, leading to a reduction of the negative symptoms brought on by histamine
• CNS depressant
• Has antiemetic properties (H2 histamine receptors in the GI System)

Indications:

• Allergic and anaphylactic reactions involving respiratory difficulties, edema or  itching, nausea/vomiting/diarrhea 

Contraindications:

• Hypersensitivity
• Patients under the age of 12

Precautions:

• The sedative effects of Benadryl can be potentiated by the administration of CNS depressants, other antihistamines, narcotics and alcohol
• Acute asthma as it may thicken secretions

Adverse Effects:

• Drowsiness
• Dizziness
• Headaches
• Excitable state
• Thickening of bronchial secretions
• Chest tightness
• Reflex tachycardia
• Hypotension

Dose:

Adult:

• 25 - 50 mg IV, IM, IO given once.
• 25 mg for patients over 60 years of age or for extrapyramidal symptoms.

Pediatric:

• Not recommended in the pediatric population.  Parents/guardians may give their child a second-generation antihistamine such as cetirizine (Reactine) as per package directions if they have it.

• IV, IM

• Pregnancy category B [if there is a clinical need for it, Category B drugs are considered safe to use].

• Health Canada (healthycanadians.gc.ca)
• Compendium of Pharmaceuticals and Specialties (CPS)
• Pharmacology for the Prehospital Professional 2^nd Edition