Diphenhydramine

Question 1

Diphenhydramine Trade Name

Answer 1

Benadryl

Question 2

Diphenhydramine Classification

Answer 2

CT Protocols:
Antihistamine

Medscape:
Histamine H1-receptor antagonist, Antiemetic Agents; Antihistamines, 1st Generation

Question 3

Diphenhydramine Mechanism

Answer 3

Statpearls:
Diphenhydramine mainly works through antagonizing the H1 (histamine 1) receptor. The H1 receptor is located on respiratory smooth muscles, vascular endothelial cells, the gastrointestinal tract (GIT), cardiac tissue, immune cells, the uterus, and the central nervous system (CNS) neurons. When the H1 receptor is stimulated in these tissues, it produces a wide variety of actions, including increased vascular permeability, promotion of vasodilation causing flushing, decreased atrioventricular (AV) node conduction time, stimulation of sensory nerves of airways producing coughing, smooth muscle contraction of bronchi and GIT, and eosinophilic chemotaxis promoting the allergic immune response.

Diphenhydramine acts as an inverse agonist at the H1 receptor, thereby reversing the effects of histamine on capillaries, reducing allergic reaction symptoms.

Question 4

Diphenhydramine Onset of Action

Answer 4

Medscape:
15-30 min

Question 5

Diphenhydramine Duration of Action

Answer 5

Medscape:
≤12 hr (histamine-induced flare suppression);
≤10 hr (histamine-induced wheal suppression)

Question 6

Diphenhydramine Indications

Answer 6

CT Protocols:
Allergic reaction, With epinephrine for anaphylaxis, Antidote for dystonic reaction/akathisia, Insomnia, Cough, Motion Sickness, Parkinsonism

Used in conjunction with epinephrine and corticosteroids for treatment of anaphylaxis

Question 7

Diphenhydramine Contraindications

Answer 7

CT Protocols:
Hypersensitivity, otherwise none in emergency setting

Question 8

Diphenhydramine Side Effects/ Precautions/ Comments

Answer 8

Capital:
Dizziness, Drowsiness, Confusion, Hypotension, Nausea/vomiting, Dry mouth

Question 9

Diphenhydramine Adult Dose

Answer 9

CT Protocols:
Allergic Reaction/Anaphylaxis: 25-50mg IV/IO/IM/PO

Dystonia/Behavioral Emergencies: 20-50 mg IV/IO or 50mg IM

Pain Management (migraines receiving prochloroperazine): 20-50mg IV/IO/IM

Question 10

Diphenhydramine Pediatric Dose

Answer 10

Capital:
1 mg/kg IV/IO/IM/PO
Not recommended for children under 10 kg 

Question 11

Diphenhydramine
Max Dose Adult (Pediatric)

Answer 11

50mg (follow dosing protocols)

Question 12

Diphenhydramine Routes of Administration

Answer 12

IV/IO/IM/PO

Question 13

Diphenhydramine How Supplied

Answer 13

10mg/ml and 50mg/ml vials; 25mg and 50mg tablets