# Allergic reaction & anaphylaxis

Separate rash-only reactions from angioedema and systemic anaphylaxis.

Population: Adult / adolescent. Scope: ALS.
Status: published. Source-linked OCEMS summary.
Summary updated: 2026-09-16.
Source checked: 2026-09-16.
Source version: See linked source for document revision and implementation dates..
Canonical page: https://www.ocmedic.com/guides/anaphylaxis/
Official source: [SO-M-015 — Allergic Reaction / Anaphylaxis](https://www.ochealthinfo.com/sites/healthcare/files/2023-04/SO-M-15%20Allergic%20Rxn%204-2023.pdf)

Independent OC Medic summary. Does not replace the complete policy, current system notices, or clinical judgment.

## Key cautions & base contact

- Epinephrine precedes diphenhydramine in anaphylaxis.
- The IM and IV/IO epinephrine concentrations differ tenfold; confirm indication, concentration, route, and dose.

## Source documents

- [SO-M-015 — Allergic Reaction / Anaphylaxis](https://www.ochealthinfo.com/sites/healthcare/files/2023-04/SO-M-15%20Allergic%20Rxn%204-2023.pdf)

## Classify the presentation

- Rash/urticaria only, stable vital signs, and no anaphylaxis history follows the mild-reaction branch. If SpO₂ is below 95%, use the anaphylaxis branch.
- Anaphylaxis findings include hypotension, wheeze, hypoxia, stridor, intraoral swelling, GI symptoms, or impending obstruction.
- Facial/cervical angioedema alone has a separate treatment branch: one 0.5 mg IM epinephrine dose, held if an auto-injector was already given, plus oxygen and diphenhydramine as specified.

## Treat anaphylaxis promptly

- Give epinephrine 0.5 mg IM into the lateral thigh using 1 mg/mL. Count a prearrival auto-injector as the first dose.
- Give high-flow mask oxygen or nasal cannula at 6 L/min as tolerated. Establish IV/IO; for hypotension, saline 250 mL boluses up to 1 L to maintain perfusion.
- For wheeze or room-air hypoxia below 95%, give albuterol 5 mg/6 mL by continuous nebulization as tolerated.

## Persistent severe symptoms

- After 5 minutes, continued hypotension, respiratory distress, or impending airway obstruction permits a second 0.5 mg IM epinephrine dose OR 0.3 mg IV/IO using 0.1 mg/mL.
- Give diphenhydramine 50 mg IM/IV once after initial epinephrine; do not give it if taken before arrival.

## Contact & transport

- Anaphylaxis requires base contact and ALS escort to a base-designated ERC. Reassess airway, breathing, BP, and recurrence after every intervention.
- Patients who self-treated with an epinephrine auto-injector still require ALS escort and evaluation even when symptoms are resolving.