Key cautions & base contact

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

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.

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.

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.

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.