Diphenhydramine in Prehospital Care
Diphenhydramine (Benadryl) is a first-generation antihistamine that blocks H1 histamine receptors. In EMS, it is primarily used for allergic reactions and drug-induced dystonic reactions. While it is a familiar over-the-counter medication, it has significant side effects and drug interactions that EMS providers must understand.
Mechanism of Action
Diphenhydramine competitively blocks H1 histamine receptors, preventing histamine from causing vasodilation, increased vascular permeability, bronchoconstriction, and pruritus (itching). It also has anticholinergic properties (which contribute to both its therapeutic effects and side effects), mild sedative effects, and some antiemetic activity.
Prehospital Indications
Allergic Reactions
Diphenhydramine is used as an adjunct in allergic reactions to manage urticaria (hives), itching, and angioedema. For anaphylaxis, epinephrine is always the first-line treatment. Diphenhydramine is a second-line medication that helps control residual symptoms after epinephrine has been administered.
Dystonic Reactions
Drug-induced dystonic reactions (involuntary muscle contractions causing abnormal postures, typically of the face, neck, and eyes) are caused by dopamine-blocking medications such as haloperidol, metoclopramide, and prochlorperazine. Diphenhydramine rapidly reverses these reactions through its anticholinergic properties.
Dosing
- Adult: 25 to 50 mg IV, IM, or orally
- Pediatric: 1 to 1.25 mg/kg IV, IM, or orally (maximum 50 mg)
- IV administration: Administer slowly over 1 to 2 minutes to minimize hypotension
Important Cautions
First-generation antihistamines like diphenhydramine cross the blood-brain barrier, causing significant sedation. This is a concern in EMS because sedation can complicate ongoing patient assessment, mask neurological findings, and contribute to respiratory depression when combined with other CNS depressants.
Additional cautions include:
- Anticholinergic effects: Dry mouth, urinary retention, blurred vision, tachycardia, and in elderly patients, confusion and delirium
- QT prolongation: At high doses, diphenhydramine can prolong the QT interval
- Drug interactions: Additive sedation with opioids, benzodiazepines, and alcohol
- Elderly patients: Higher risk of anticholinergic toxicity, falls, and confusion
- Overdose potential: Diphenhydramine overdose can cause seizures, rhabdomyolysis, and cardiac arrhythmias
For the NREMT exam, remember that diphenhydramine is an adjunct treatment for allergic reactions, not a replacement for epinephrine in anaphylaxis. Understand its anticholinergic side effect profile and the risks of combining it with other CNS depressants.