Anaphylaxis
Lie the patient flat, give IM adrenaline into the thigh straight away, repeat after 5 minutes if needed, and give fluids for low BP. Antihistamines and steroids come later, not first.
Ten minutes after starting IV ceftriaxone, a 45-year-old woman feels her throat closing. She has hives on her chest, a hoarse voice and wheeze. BP 78/40, pulse 130.
What do you do in the next two minutes?
- Stop the triggerStop the infusion or drug. Call for help.
- Lie the patient flatLegs raised. Sit up only if breathing is the main problem and BP is normal. Never stand them up.
- IM adrenaline now0.5 mg (0.5 mL of the 1 mg/mL ampoule) into the outer thigh.
- Repeat after 5 minutesIf no improvement.
- Oxygen and IV accessHigh-flow oxygen. Two cannulas.
- Fluid bolus for low BP500–1000 mL crystalloid, repeat as needed.
- Still not better after two dosesRefractory anaphylaxis: senior and ICU now.
- Then the restNon-sedating antihistamine once stable. Tryptase sample. Observe.
- Any suspected anaphylaxis: call for help immediately
- Stridor, hoarse voice or swelling of tongue or throat
- Low BP not responding to IM adrenaline and fluids
- Still unwell after two doses of IM adrenaline
- On beta-blockers (may need glucagon)
- Cardiac arrest
- Airway swelling
- Biphasic reaction hours later
- Anaphylaxis presenting as asthma or collapse alone
- Delayed adrenaline while giving antihistamine and steroid
- Angioedema from ACE inhibitors (not anaphylaxis)
| First drug | IM adrenaline into the thigh. Nothing else comes first |
| IM adrenaline (adult) | 0.5 mg = 0.5 mL of the 1 mg/mL (1:1000) ampoule |
| Repeat | Every 5 minutes if no improvement |
| Fluids | 500–1000 mL crystalloid bolus for low BP |
| IV adrenaline | Only by an experienced senior, as an infusion, with monitoring. Never a 1 mg IV bolus outside cardiac arrest |
Check every dose against a current drug reference and your hospital protocol.
On this page
What anaphylaxis is
Anaphylaxis is a severe, life-threatening allergic reaction that develops over minutes. Mast cells release histamine and other chemicals, making blood vessels leak and widen and airways swell and tighten. The patient can die from airway swelling, bronchospasm or shock.
Common triggers in hospital: antibiotics (especially IV), contrast dye, anaesthetic drugs, blood products, NSAIDs. Outside hospital: foods (nuts, seafood), wasp and bee stings, medicines.
Sudden onset and rapid progression, plus life-threatening problems with:
- Airway: swelling of throat or tongue, hoarse voice, stridor, or
- Breathing: wheeze, breathlessness, low SpO₂, tiredness, or
- Circulation: low BP, pale and clammy, fainting, drowsiness, collapse.
Skin changes (hives, flushing, swelling) are common but can be absent, so do not wait for a rash. Skin changes alone, without A, B or C problems, are not anaphylaxis.
The adrenaline rules
- Adrenaline first. Not antihistamine, not hydrocortisone. In many emergency departments the "allergy cocktail" of chlorphenamine and hydrocortisone is given first and the adrenaline comes late. That delay kills.
- Route: intramuscular, into the outer middle thigh (through clothing if needed). It is fast and safe.
- Dose: 0.5 mg in an adult = 0.5 mL of the 1 mg/mL (1:1000) ampoule. In Pakistan, auto-injectors are often not available, so draw it up from the ampoule with a 1 mL syringe.
- Repeat every 5 minutes if the patient is not improving.
- Never give 1 mg of adrenaline as an IV push to a patient with a pulse. That is the cardiac arrest dose. It can cause fatal arrhythmias, heart attack and severe hypertension.
- 500 micrograms (0.5 mL)
- 300 micrograms if the child is small or prepubertal
300 micrograms (0.3 mL)
150 micrograms (0.15 mL)
100–150 micrograms (0.1–0.15 mL)
Step by step
- Recognise and call for helpShout for help. Note the time.
- Remove the triggerStop the drug, infusion or blood.
- PositionLie flat with legs raised. If breathing is difficult and BP is fine, let them sit up. Pregnant: lie on the left side. Never stand or sit up a shocked patient: they can die from sudden loss of venous return.
- IM adrenaline0.5 mg into the thigh.
- OxygenHigh flow, 15 L/min through a non-rebreather mask.
- IV access and fluidsTwo cannulas. 500–1000 mL bolus if BP is low.
- MonitorSpO₂, ECG, BP every few minutes.
- Reassess at 5 minutesNot improving: repeat IM adrenaline.
- After two IM doses and still unwellRefractory anaphylaxis: senior and ICU now.
Refractory anaphylaxis
If the patient is still unwell after two doses of IM adrenaline and fluids:
- Senior, anaesthetist and ICU now.
- Keep giving IM adrenaline every 5 minutes until an adrenaline infusion is running.
- IV adrenaline infusion is started by an experienced senior, with continuous ECG and BP monitoring.
- Treat bronchospasm with nebulised salbutamol and ipratropium.
- On beta-blockers? Adrenaline may not work well. Glucagon is the senior's option.
- Prepare for a difficult airway if there is swelling of the throat.
Antihistamines and steroids
- Antihistamines treat the itching and hives. They do not treat airway swelling, bronchospasm or shock. Give a non-sedating antihistamine (such as cetirizine) once the patient is stable. Chlorphenamine IV can cause a drop in BP if given fast.
- Steroids are no longer routinely recommended for the emergency treatment of anaphylaxis (Resuscitation Council UK 2021). They may help if asthma symptoms continue: a senior decision.
- Neither of these is ever a substitute for adrenaline.
Tests and observation
- Mast cell tryptase: take a blood sample as soon as possible after starting treatment, a second at 1–2 hours, and a third at 24 hours or at follow-up. It helps confirm the diagnosis later. Do not delay treatment to take it.
- Observe: at least 6 hours after symptoms resolve if they responded to one dose of adrenaline; at least 12 hours if they needed two or more doses, had a severe reaction, a previous biphasic reaction, or arrived at night or live far away.
- Biphasic reactions: symptoms can return hours later without new exposure.
Before discharge
- Explain what happened and what the trigger probably was. Write it clearly in the notes and on the drug chart: allergy, reaction, date.
- Adrenaline auto-injector if available and appropriate. If not, teach the family what to do and to come straight to hospital.
- Avoid the trigger. Warn about related drugs (for example, other beta-lactams after a severe penicillin reaction, until assessed).
- Referral to an allergy or immunology clinic where available.
- Medical alert card or bracelet.
Angioedema that is not anaphylaxis
Swelling of the lips, tongue or face without hives, often in a patient on an ACE inhibitor (ramipril, enalapril, lisinopril) or with a family history. It is caused by bradykinin, not histamine, so adrenaline, antihistamines and steroids may not work well. The airway is still at risk: call your senior and the anaesthetist early. Stop the ACE inhibitor for good.
Common mistakes
- Giving chlorphenamine and hydrocortisone first and adrenaline late (or not at all).
- Giving adrenaline under the skin instead of into the muscle.
- Giving 1 mg of adrenaline IV to a patient who has a pulse.
- Sitting a shocked patient up or walking them to the bathroom.
- Sending the patient home after an hour because they look well.
- Not documenting the allergy clearly.
Frequently asked questions
What is the dose of adrenaline in anaphylaxis?
For adults, 0.5 mg intramuscularly (0.5 mL of the 1 mg/mL, 1:1000 ampoule) into the outer thigh, repeated after 5 minutes if there is no improvement.
Why is adrenaline given IM and not IV in anaphylaxis?
Intramuscular adrenaline is fast, effective and safe. IV boluses can cause dangerous arrhythmias and very high BP. IV adrenaline is only given as a carefully monitored infusion by experienced staff, except in cardiac arrest.
Are steroids needed in anaphylaxis?
They are no longer routinely recommended in the emergency treatment of anaphylaxis. They do not act quickly enough to help in the first hours and must never delay adrenaline.
How long should a patient be observed after anaphylaxis?
At least 6 hours after symptoms resolve if one dose of adrenaline worked, and at least 12 hours after severe reactions, two or more doses, or a previous biphasic reaction.
Test yourself
Exam-style questions on this guide, from our two question banks. Choose an answer to see why each option is right or wrong.
1. A 35-year-old develops hives, wheeze and BP 80/45 minutes after IV co-amoxiclav. The nurse has drawn up chlorphenamine and hydrocortisone. What should be given first?
Answer: IM adrenaline 0.5 mg. IM adrenaline is the first and most important drug. Adult dose: 0.5 mg (0.5 mL of 1 mg/mL, 1:1000). Repeat after 5 minutes if not improving.
- Nebulised salbutamol: Does not treat the shock.
- IV chlorphenamine 10 mg: Antihistamines do not treat airway or circulation problems.
- IV adrenaline 1 mg bolus: IV bolus adrenaline is for cardiac arrest only.
- IV hydrocortisone 200 mg: Steroids act too slowly and are not first-line.
2. Five minutes after IM adrenaline for anaphylaxis, the BP is still 82/50. What is the next step?
Answer: Repeat IM adrenaline and give a fluid bolus. Repeat IM adrenaline every 5 minutes if needed, with a 500–1000 mL crystalloid bolus. After two doses without improvement, refractory anaphylaxis needs an adrenaline infusion with critical care.
- Start oral prednisolone: No role here.
- Give chlorphenamine and wait 15 minutes: Doesn't treat the shock.
- Give IV hydrocortisone and wait: Steroids do not treat refractory shock.
- Observe for 1 hour: Not responding needs further treatment now.
3. A 60-year-old woman on ramipril has a swollen tongue and lips. There is no rash or itch. What is the most likely cause?
Answer: ACE-inhibitor (bradykinin) angioedema. ACE inhibitors cause bradykinin angioedema, often without urticaria. It responds poorly to adrenaline and antihistamines. The airway is still at risk: involve the anaesthetist early and stop the ACE inhibitor permanently.
- Acute urticaria: No hives.
- Hereditary angioedema new at age 60: Unlikely to present first at this age.
- Food allergy: No exposure history and no rash.
- Contact dermatitis: Not this picture.
4. A 22-year-old has recurrent swelling of the lips and abdomen without urticaria or itch since adolescence. Antihistamines and adrenaline don’t help. Low C4 is found. What is deficient?
Answer: C1 esterase inhibitor. Hereditary angioedema (autosomal dominant, C1-INH deficiency or dysfunction) causes bradykinin-mediated non-itchy swelling of skin, gut (abdominal pain) and larynx (life-threatening), without urticaria. Triggers: trauma, dental work, oestrogens, ACE inhibitors. C4 is low between attacks. Acute attacks: C1-INH concentrate, icatibant (bradykinin B2 antagonist); FFP if unavailable. Antihistamines, steroids and adrenaline are ineffective.
- IgE: IgE-mediated angioedema comes with urticaria and responds to adrenaline.
- Complement C3: C3 is not the deficient factor in hereditary angioedema.
- Histamine N-methyltransferase: This enzyme is not linked to bradykinin angioedema.
5. A 55-year-old who started lisinopril 2 months ago presents with swelling of the lips and tongue without urticaria or itching; airway is patent. What is the most important step?
Answer: Stop the ACE inhibitor permanently. ACE-inhibitor angioedema: bradykinin-mediated (reduced breakdown) — lips, tongue, face, airway; no urticaria/pruritus; can occur months–years after starting; commoner in people of African ancestry, smokers. Responds poorly to antihistamines/steroids/adrenaline (may still be given if airway threatened); icatibant or FFP in severe cases; airway management. Stop ACE inhibitor permanently; ARBs carry a small risk (use cautiously); avoid sacubitril–valsartan. Histamine angioedema: with urticaria, itch — antihistamines.
- Switch to another ACE inhibitor: ACE inhibitor angioedema is a class effect.
- Continue and add antihistamine: Continuing risks recurrence.
- Give penicillin: This is not infective.
6. At low doses, adrenaline dilates skeletal muscle arterioles. Which receptor mediates this?
Answer: β2. β2 receptors are more sensitive to adrenaline than α1 receptors. At low doses, β2 effects dominate in skeletal muscle vessels (dilation), so diastolic pressure may fall. At high doses, α1 constriction dominates and blood pressure rises.
- α1: Constricts; dominates at high doses.
- β1: Heart rate and contractility.
- α2: Presynaptic inhibition.
- D1: Renal and mesenteric dilation (dopamine).
7. Minutes after an injection of penicillin, a patient develops wheeze, swelling of the lips and a blood pressure of 70/40 mmHg. What is the first drug to give?
Answer: Adrenaline 0.5 mg intramuscularly. This is anaphylaxis. The first and most important treatment is intramuscular adrenaline 0.5 mg (0.5 mL of 1 mg/mL) into the anterolateral thigh, repeated after 5 minutes if needed. Adrenaline reverses vasodilatation, bronchospasm and oedema. Steroids and antihistamines are not first-line.
- Hydrocortisone 200 mg intravenously: Slow to act; may help prevent a biphasic reaction but does not treat the acute reaction.
- Chlorphenamine 10 mg intravenously: Treats itching and urticaria only; not life-saving.
- Salbutamol nebuliser: Helps wheeze but does nothing for shock or airway swelling.
- Adrenaline 1 mg intravenous bolus: The 1 mg IV dose is for cardiac arrest; it can cause fatal arrhythmias in a patient with a pulse.
References and dose sources
- Resuscitation Council UK: Emergency treatment of anaphylaxis (2021)
- World Allergy Organization anaphylaxis guidance
- NICE CG134 — anaphylaxis: assessment and referral
- Adrenaline (epinephrine) dosing: Drugs.com: Epinephrine dosage (see also our adrenaline (epinephrine) dose page)
- Sodium chloride (normal saline, 3% saline) dosing: Drugs.com: Sodium chloride dosage (see also our sodium chloride (normal saline, 3% saline) dose page)
- Cetirizine dosing: Drugs.com: Cetirizine dosage (see also our cetirizine dose page)
- Salbutamol dosing: Drugs.com: Albuterol (salbutamol) dosage (see also our salbutamol dose page)
International guidance is linked where it reflects good practice. Where your hospital’s protocol differs, follow your hospital.