Anaphylaxis is a favourite exam emergency because the single right action — intramuscular adrenaline — is so often delayed in real life, and because the 2021 Resuscitation Council UK update changed long-taught habits. This guide covers the essentials to that standard. Follow current guidance and local protocols in practice; this reflects Resuscitation Council UK guidance as of mid-2026.
What anaphylaxis is
Anaphylaxis is a severe, life-threatening systemic hypersensitivity reaction. It is a clinical diagnosis defined by sudden onset, rapid progression, and life-threatening Airway, Breathing or Circulation problems — usually, though not always, with skin and mucosal changes such as urticaria, flushing or angio-oedema. Crucially, skin changes alone are not anaphylaxis, and gastrointestinal symptoms without Airway, Breathing or Circulation involvement do not usually indicate it, though they can with a venom sting. It sits at the severe end of a spectrum of allergic reactions, distinguished from a milder reaction by the presence of those life-threatening airway, breathing or circulatory features.
Recognising it
Look for the combination of a likely trigger (food, drug, venom, contrast), a rapid onset over minutes, and at least one of: airway swelling (throat tightness, stridor, a hoarse voice), breathing difficulty (wheeze, hypoxia, fatigue), or circulatory compromise (hypotension, tachycardia, collapse). The diagnosis is clinical — treatment must not wait for tests. The common triggers in the UK are foods (nuts, shellfish, milk, egg), drugs (antibiotics, particularly penicillins, and non-steroidal anti-inflammatories), insect venom, and contrast media, with a proportion idiopathic. Most reactions are IgE-mediated — sensitising on first exposure and reacting on re-exposure — but the mechanism does not change the immediate treatment. Onset is usually within minutes to a couple of hours of exposure, and the faster the onset, the more severe the reaction tends to be.
Management to the Resuscitation Council UK standard
Adrenaline is the mainstay, and giving it early is what saves lives.
Intramuscular adrenaline first: the moment anaphylaxis is recognised, give intramuscular adrenaline into the anterolateral thigh. The adult and teenager dose is 500 micrograms (0.5 mL of 1 in 1000). Repeat after 5 minutes if Airway, Breathing or Circulation features do not improve. Doses are weight- and age-based in children.
Position and resuscitate: lie the patient flat to support the circulation — or sit them up only if breathing is easier — and do not let them stand or sit up suddenly, as this can precipitate cardiac arrest. Give high-flow oxygen and an intravenous fluid bolus for circulatory compromise, and treat with an ABCDE approach. Adrenaline can be repeated every 5 minutes as needed, and most patients respond to one or two intramuscular doses.
Refractory anaphylaxis: where there is no improvement after two appropriate doses of intramuscular adrenaline, the reaction is refractory and needs an intravenous adrenaline infusion given by experienced clinicians, continued fluids, and early critical-care involvement.
What changed in 2021
This is heavily examined. Corticosteroids such as hydrocortisone are no longer recommended for the routine emergency treatment of anaphylaxis, and antihistamines are no longer part of the acute algorithm — they may help cutaneous symptoms later but do nothing for the life-threatening features and must never delay or replace adrenaline. The emphasis is firmly on early, repeated intramuscular adrenaline and fluids. The rationale is that adrenaline reverses the airway, breathing and circulatory features through its actions on the heart and blood vessels, whereas steroids act too slowly to help the acute event and the evidence that they prevent biphasic reactions is weak.
After the acute episode
Take a timed serum mast cell tryptase as soon as feasible after treatment, with a repeat sample, to support the diagnosis. Observe the patient afterwards — the duration depends on severity and the risk of a biphasic reaction, with longer observation for severe reactions and for children. On discharge, prescribe adrenaline auto-injectors, train the patient and carers in their use, and refer to a specialist allergy clinic. A biphasic reaction — a recurrence hours after apparent recovery, without further exposure — is the reason for a period of observation, and patients must know to use their auto-injector and call for help if symptoms return after they go home.
High-yield exam points and traps
- Intramuscular adrenaline is always the immediate priority, given without delay — not intravenous adrenaline as a first step outside experienced hands.
- The adult dose is 500 micrograms IM (0.5 mL of 1 in 1000); repeat after 5 minutes if there is no improvement.
- Steroids and antihistamines are not part of the acute 2021 algorithm — a frequent update trap; they are at most adjuncts after resuscitation.
- Lying the patient flat matters: sudden standing can be fatal.
- Refractory anaphylaxis (no response after two IM doses) needs an IV adrenaline infusion and critical care.
- Discharge is not complete without auto-injectors, training and allergy referral.
- Auto-injectors deliver a maximum of 300 micrograms, so in a healthcare setting the 500-microgram dose is best drawn up from an ampoule.
A few common questions
What is the immediate treatment for anaphylaxis? Intramuscular adrenaline into the anterolateral thigh — 500 micrograms for adults and teenagers — repeated after 5 minutes if there is no improvement.
Are steroids and antihistamines used in anaphylaxis? Not in the acute 2021 algorithm; corticosteroids are no longer routinely recommended and antihistamines are not part of acute management, though they may ease later skin symptoms.
What is refractory anaphylaxis? Anaphylaxis with no improvement after two appropriate doses of intramuscular adrenaline; it requires an intravenous adrenaline infusion by experienced clinicians and critical care, carries a higher mortality, and needs early escalation.
Why must the patient not stand up? Sudden standing or sitting in anaphylaxis can cause fatal circulatory collapse, so patients are kept lying flat, or semi-recumbent if breathing is easier.
What happens after the acute treatment? Timed tryptase samples, a period of observation for biphasic reactions, and discharge with auto-injectors, training and allergy referral.
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