Associated with adverse drug interactions - anaphylaxis

Red flags

Airway swelling, stridor, wheeze, hypotension, or reduced consciousness after a drug exposure
Biphasic reaction risk - recurrence hours after apparent resolution
Any hypotension/respiratory compromise following an IV drug/contrast/NMBA administration in theatre

IgE-mediated (immediate, immunological)

Beta-lactam antibiotics (commonest drug cause), neuromuscular blocking agents (commonest perioperative cause), biologics (monoclonal antibodies)

Non-IgE mast cell/basophil activation

NSAIDs/aspirin (direct COX inhibition, cross-reactive across the class), radiocontrast media, opioids (direct mast cell degranulation), vancomycin ("red man syndrome" - rate-related, not true allergy)

Complement/bradykinin-mediated (not true anaphylaxis but overlaps clinically)

ACE-inhibitor angioedema - no urticaria, poor response to adrenaline/antihistamines, can involve larynx

Exact temporal relationship to drug administration (minutes for IgE-mediated)
Prior reactions to the same or related drug class, cross-reactivity risk (e.g., penicillin/cephalosporin)
Route of administration (IV higher risk of rapid severe reaction than oral)

Airway, breathing, circulation - stridor, wheeze, hypotension, urticaria, angioedema
Distinguish from vasovagal (bradycardia, no urticaria/respiratory compromise) and ACE-I angioedema (no urticaria)

Serum tryptase - timed samples (at presentation, 1-2h, and baseline at 24h) supports retrospective diagnosis
Skin prick/specific IgE testing or drug challenge (allergy specialist, delayed, not acute)

1. IM adrenaline - first-line, immediate

0.5mg (0.5mL of 1:1000) IM into the anterolateral thigh, repeated every 5 minutes as needed
Do not substitute antihistamines/corticosteroids/nebulised salbutamol for adrenaline

2. Supportive

High-flow oxygen, IV fluids for hypotension, supine/legs-raised positioning (unless respiratory distress)
Antihistamines and corticosteroids are adjuncts only, not first-line treatment

3. Observation

Minimum 4 hours observation post-reaction for biphasic reaction risk (longer if severe/refractory)
Refer to immunology/allergy for confirmatory testing and an anaphylaxis action plan
Prescribe an adrenaline autoinjector and document the allergy clearly

4. ACE-inhibitor angioedema

Stop the ACE-inhibitor permanently; adrenaline/antihistamines often ineffective; icatibant (bradykinin receptor antagonist) or C1-esterase inhibitor concentrate for severe cases

Adrenaline is never contraindicated in true anaphylaxis - hesitation to give it is the commonest management failure
ACE-inhibitor angioedema does not reliably respond to the standard anaphylaxis algorithm - different mechanism, different treatment
Vancomycin "red man syndrome" is rate-related histamine release, not a true allergy - manage by slowing the infusion, not by labelling a lifelong allergy
Beta-lactam allergy labels are frequently inaccurate and unnecessarily restrict future antibiotic choice - refer for allergy testing/de-labelling where appropriate

1. Adrenaline first, always

2. Recognise the bradykinin mimic

"Angioedema with no urticaria on an ACE-inhibitor doesn't respond to the same algorithm - I stop the drug and consider icatibant."

7 more sections, plus exam facts

Premium unlocks every note across every specialty, and the full exam fact library behind it.