Allergic Reaction

at the bedside

airway, breathing, circulation; full vitals, cardiac monitor, IV access; stop the suspected drug or infusion
anaphylaxis → skin or mucosal signs plus airway, breathing or circulation compromise, or hypotension after a known allergen: epinephrine 0.5 mg IM now, before anything else → anaphylaxis
lie flat with legs raised; sitting up or standing can precipitate arrest; high-flow oxygen
airway → stridor, voice change, tongue or lip swelling: anesthesia and ENT early

escalate now if

stridor, hoarse voice, tongue or lip swelling → anesthesia and ENT now, prepare for intubation → epinephrine
wheeze, hypoxia, hypotension or collapse with a rash → anaphylaxis, epinephrine IM now
no response to two doses of epinephrine → refractory, ICU now
on a beta-blocker and not responding → glucagon
angioedema without urticaria on an ACE inhibitor → ACEi angioedema, epinephrine and antihistamines work poorly

anaphylaxis → minutes after exposure; skin plus respiratory, cardiovascular or severe GI involvement
urticaria or angioedema alone → skin only, normal vitals, no airway symptoms
ACE inhibitor angioedema → lips, tongue, face; no itch or urticaria; any time on the drug, bradykinin-mediated
drug reaction with systemic features → fever, rash, eosinophilia days to weeks in (DRESS); mucosal blistering (SJS/TEN) → stop the drug, dermatology → new rash
not allergy → vasovagal, asthma attack, panic, red man syndrome with fast vancomycin, scombroid, septic shock

trigger → drug just given (antibiotic, contrast, NSAID, neuromuscular blocker), transfusion, food, sting; minutes from exposure to symptoms
history → previous reactions and what treated them, asthma (higher risk of a severe reaction), beta-blocker or ACE inhibitor
exam → urticaria, flushing, angioedema; stridor, wheeze, work of breathing; BP trend and perfusion; vomiting, abdominal pain

diagnostic criteria

acute onset with skin or mucosal involvement plus respiratory compromise, hypotension or severe GI symptoms
or acute hypotension, bronchospasm or laryngeal involvement after a known or probable allergen, even without skin signs
hypotension → SBP < 90 mmHg or a drop ≥ 30% from baseline

nothing before epinephrine; bloods after the first dose
serum tryptase → at 1–3 h, supports the diagnosis later; a normal level does not exclude it
VBG, ECG → if hypotensive or hypoxic; troponin if chest pain or persistent hypotension
transfusion reaction suspected → transfusion reaction

anaphylaxis

epinephrine 0.5 mg IM (0.5 mL of 1 mg/mL) mid-outer thigh; repeat every 5 min while airway, breathing or circulation is not improving
lie flat, legs raised; high-flow oxygen; remove the trigger
crystalloid 500–1000 mL IV fast, reassess, repeat; smaller boluses if heart failure
wheeze → salbutamol 2.5–5 mg neb after the epinephrine, not instead of it
observe 6–24 h on the monitor; biphasic reactions happen

after the emergency → cetirizine 10 mg PO or chlorphenamine 10 mg IV for the itch; steroids are not routine (hydrocortisone 200 mg IV only for refractory reaction or concurrent asthma); neither prevents a biphasic reaction or replaces epinephrine
document the trigger as an allergy in the chart tonight

refractory: no response to two IM doses

ICU now; epinephrine infusion 1–10 mcg/min, titrated by ICU, vasopressors if needed → vasopressors & inotropes
on a beta-blocker → glucagon 1–2 mg IV over 5 min, repeat or infuse
airway swelling progressing → early intubation by anesthesia; surgical airway backup

ACE inhibitor angioedema

stop the ACE inhibitor for good; airway first, anesthesia and ENT early
epinephrine, antihistamines and steroids are given if anaphylaxis cannot be excluded, but rarely help; icatibant or C1 inhibitor are specialist options where available

urticaria or mild angioedema, normal vitals

cetirizine 10 mg PO or loratadine 10 mg PO; stop the trigger; reassess in 30 min for progression

hand over → trigger, epinephrine doses and times, observation period, allergy documented, escalation trigger

source	type
2023 AAAAI/ACAAI anaphylaxis practice parameter	international guideline
2021 Resuscitation Council UK anaphylaxis guideline (steroids not routine)	international guideline

Verify indication, dose, allergies, interactions, renal/hepatic function and local protocols before prescribing.

free the on-call checklist covers the whole shift on one printable page.

studying for the IM exam? the IM Rapid Review covers this in the same format. see the sample chapter.

reviewed Jun 2026updated Oct 2026file complaint/allergic-reaction