Diphenhydramine

Provider Level

AEMT/RN/Paramedic

Class

Antihistamine (H1-receptor antagonist), Anticholinergic

Mechanism of Action

Competes with free histamine for binding at H1 receptor sites in the GI tract, blood vessels, and respiratory tract, preventing histamine-induced responses. Also possesses significant anticholinergic and sedative properties.

Indications

Symptomatic relief of allergic symptoms, adjunct to epinephrine in anaphylaxis, treatment of extrapyramidal symptoms (dystonic reactions), and relief of severe motion sickness.

Contraindications

Hypersensitivity to the drug, neonates or premature infants, nursing mothers. Often contraindicated or used with extreme caution in narrow-angle glaucoma, symptomatic prostatic hypertrophy, and acute asthma exacerbations due to the thickening of bronchial secretions.

Routes

PO, IM, IV, IO

Onset / Duration

Onset: 15 to 30 minutes for PO/IM, 1 to 5 minutes for IV/IO. Duration: 4 to 8 hours.

Adverse Effects

Sedation, drowsiness, dizziness, confusion, dry mouth and throat, thickened bronchial secretions, blurred vision, urinary retention, hypotension, tachycardia, palpitations, and paradoxical CNS excitation (particularly in pediatric patients).

Precautions & Interactions

Use with caution in patients with asthma, cardiovascular disease, hypertension, or increased intraocular pressure. May cause profound sedation; effects are potentiated by alcohol and other CNS depressants. IV administration should be slow to avoid hypotension.

Special Considerations

Authorized for AEMT, RN, and Paramedic levels. Diphenhydramine is not a first-line medication for anaphylaxis and must not replace epinephrine. Elderly patients are particularly sensitive to anticholinergic side effects and risk of falls.