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Anaphylaxis: Symptoms & Emergency Treatment

Anaphylaxis is a severe, life-threatening allergic reaction that can progress within minutes. Recognising symptoms and using adrenaline promptly saves lives.

Onset
Minutes to 2 hours after exposure
Treatment
Adrenaline (epinephrine) — first-line, immediate
Biphasic reaction risk
Up to 20% — can recur hours later
Common triggers
Food, insect stings, medications, latex

Recognising Anaphylaxis

SystemSymptoms
SkinHives, flushing, swelling (especially face, lips, tongue)
RespiratoryWheeze, difficulty breathing, throat tightness, hoarse voice
CardiovascularDizziness, fainting, rapid heartbeat, low blood pressure (shock)
GastrointestinalNausea, vomiting, abdominal cramping
GeneralSense of impending doom, anxiety

Emergency Treatment Steps

  1. Call 999/911 immediately
  2. Use adrenaline auto-injector (EpiPen, Jext, Emerade) into outer mid-thigh, through clothing if needed — hold for 3 seconds
  3. Lie the person flat with legs raised (unless breathing difficulty — then sit up)
  4. Give a second dose after 5-15 minutes if no improvement
  5. Continue to hospital even if symptoms improve — observation needed for biphasic reactions

Biphasic Reactions

Up to 20% of anaphylaxis cases have a second wave of symptoms 1-72 hours later (biphasic reaction), even without further allergen exposure. This is why hospital observation (typically 6-12 hours minimum) is recommended after any anaphylaxis episode.
Always Use Adrenaline First — Don't DelayIf anaphylaxis is suspected, use the adrenaline auto-injector immediately — do not wait to see if symptoms improve, and do not rely on antihistamines alone, which are too slow-acting and insufficient for a severe reaction. Delayed adrenaline use is associated with worse outcomes.
Always Carry Two Auto-InjectorsPeople with a history of anaphylaxis should carry two adrenaline auto-injectors at all times, as a second dose is sometimes needed if symptoms don't improve or return. Check expiry dates regularly and replace promptly.
Can anaphylaxis happen without a known allergy?
Yes — a first-ever exposure to a new allergen can trigger anaphylaxis, and some cases have no identifiable trigger (idiopathic anaphylaxis).
Is it safe to give adrenaline if I'm not sure it's anaphylaxis?
Yes — adrenaline auto-injectors are designed to be very safe even if given unnecessarily; the risk of not treating true anaphylaxis far outweighs any risk from using adrenaline when it turns out not to have been needed.
How long should I stay in hospital after anaphylaxis?
Current guidelines typically recommend observation for at least 6 hours after resolution of symptoms, and longer (up to 24 hours) for severe reactions, due to the risk of biphasic reactions.
AllergiesComplete allergy guideIgE Allergy TestIdentifying triggersAsthmaRelated respiratory emergency
Medical Disclaimer: This page is for general education only and does not replace professional medical advice. Always consult a qualified healthcare provider.