Severe allergic reaction (anaphylaxis)¶
Emergency numbers differ by country. This page uses 911, the number for the United States and Canada. Many other countries use 112, 999, or another number. Find the number for your country.
Anaphylaxis is an acute, life-threatening systemic hypersensitivity reaction that develops rapidly following exposure to an allergen. Triggered by the massive, explosive degranulation of mast cells and basophils, anaphylaxis causes widespread vasodilation, capillary leakage, bronchoconstriction, and laryngeal edema. Left untreated, severe anaphylaxis can progress from mild cutaneous itching to complete upper airway occlusion or distributive cardiovascular collapse in less than 15 minutes.
This guide provides operational emergency procedures based on clinical directives from the World Allergy Organization (WAO), the National Institute of Allergy and Infectious Diseases (NIAID), the American Academy of Allergy, Asthma and Immunology (AAAAI), and the European Academy of Allergy and Clinical Immunology (EAACI) to identify multi-system symptoms, administer intramuscular epinephrine without delay, prevent fatal postural collapse, and manage biphasic recurrences.
Do this now¶
If an individual exhibits symptoms of an acute severe allergic reaction:
- Administer an epinephrine auto-injector immediately into the outer mid-thigh muscle. Epinephrine is the only first-line medication proven to stop and reverse anaphylaxis. Do not delay epinephrine administration to give oral antihistamines, asthma inhalers, or steroids.
- Call your local emergency number immediately (911 in the United States and Canada, 112 in the European Union, 999 in the United Kingdom). Explicitly inform the emergency dispatcher: "The patient is experiencing severe anaphylaxis and has received an epinephrine injection."
- Lay the person completely flat on their back and elevate their legs approximately 30 centimeters (12 inches) to maintain venous blood return to the heart. Do not allow the person to stand, walk, or sit up abruptly, as sudden upright posture can trigger fatal cardiac arrest from empty vena cava syndrome.
- If the person is experiencing severe respiratory distress or vomiting, allow them to sit semi-reclined or turn them onto their side in the recovery position to prevent airway aspiration, but keep physical exertion to zero.
- If symptoms fail to improve or worsen after 5 to 15 minutes, administer a second epinephrine auto-injector in the opposite thigh.
- If the person becomes unresponsive and ceases breathing normally, begin CPR immediately and deploy an automated external defibrillator (AED).
Anaphylaxis clinical triage flowchart¶
EXPOSURE TO KNOWN OR SUSPECTED ALLERGEN
│
Does the patient exhibit ANY of these?
- Swelling of tongue, lips, uvula, or throat?
- Wheezing, stridor, or severe shortness of breath?
- Dizziness, confusion, pallor, or fainting?
- Widespread hives PLUS vomiting/cramping?
│
┌────────────────┴────────────────┐
YES NO
│ │
┌────────┴────────┐ ┌────────┴────────┐
│ ANAPHYLAXIS │ │ LOCAL REACTION │
│ MEDICAL CRISIS │ │ (Mild / Hives) │
└────────┬────────┘ └────────┬────────┘
│ │
1. INJECT EPINEPHRINE NOW 1. Oral antihistamine
(Outer mid-thigh IM) 2. Cold compress on skin
2. CALL 911 / 112 / 999 3. Monitor for 2 hours
3. LAY FLAT, ELEVATE LEGS 4. If breathing changes:
4. NEVER ALLOW STANDING escalate to anaphylaxis
5. Time the injection │
│ │
Do symptoms improve within 5 to 15 minutes? │
│ │
┌──────────┴──────────┐ │
YES NO │
│ │ │
Maintain flat INJECT SECOND DOSE │
position; await in opposite thigh │
paramedic arrival Continue 911 call │
│ │ │
└──────────────┬──────┘ │
│ │
MANDATORY HOSPITAL OBSERVATION (4 TO 8 HOURS)
(To detect delayed biphasic recurrence)
The clinical diagnostic criteria for anaphylaxis¶
According to international consensus guidelines established by the World Allergy Organization (WAO) and NIAID, anaphylaxis is highly likely when any one of the following three clinical presentations occurs:
WORLD ALLERGY ORGANIZATION DIAGNOSTIC CRITERIA
┌──────────────────────┬─────────────────────────────────────────────┐
│ Diagnostic Scenario │ Clinical Presentation │
├──────────────────────┼─────────────────────────────────────────────┤
│ Scenario 1: │ Acute onset (minutes to hours) involving │
│ Acute Skin/Mucosal │ skin or mucosal tissue (hives, flushing, │
│ PLUS Systemic Signs │ swollen lips/tongue) PLUS AT LEAST ONE: │
│ │ a) Respiratory compromise (dyspnea, stridor)│
│ │ b) Reduced blood pressure (fainting, collapse│
├──────────────────────┼─────────────────────────────────────────────┤
│ Scenario 2: │ Two or more of the following occurring │
│ Rapid Multi-System │ rapidly after exposure to a LIKELY allergen:│
│ Presentation │ 1. Skin/mucosal tissue involvement │
│ │ 2. Respiratory compromise │
│ │ 3. Reduced blood pressure or collapse │
│ │ 4. Persistent gastrointestinal cramps/vomit │
├──────────────────────┼─────────────────────────────────────────────┤
│ Scenario 3: │ Reduced blood pressure or shock occurring │
│ Sudden Hypotension │ rapidly after exposure to a KNOWN allergen: │
│ │ Systolic BP < 90 mmHg or > 30% drop │
└──────────────────────┴─────────────────────────────────────────────┘
Common triggers¶
- Food allergens: Peanuts, tree nuts (walnuts, cashews, almonds), shellfish, finned fish, cow milk, eggs, wheat, and soy.
- Insect venom: Stings from honeybees, yellowjackets, hornets, wasps, and fire ants.
- Medications: Beta-lactam antibiotics (penicillins and cephalosporins), non-steroidal anti-inflammatory drugs (NSAIDs such as ibuprofen and aspirin), and neuromuscular blocking agents.
- Natural rubber latex: Medical gloves, balloons, and dental dams.
The myth of cutaneous symptoms¶
A common, dangerous misconception is that anaphylaxis must always present with widespread skin hives (urticaria). Clinical registry data indicates that up to 10 to 20 percent of fatal anaphylactic episodes present with zero skin involvement. An individual can experience fatal cardiovascular collapse or catastrophic laryngeal obstruction without developing a single hive.
Epinephrine: the only first-line life-saving medication¶
Epinephrine (adrenaline) is the physiological antidote to anaphylaxis. There is no medical substitute:
PHARMACOLOGICAL ACTIONS OF EPINEPHRINE IN ANAPHYLAXIS
┌──────────────────────┬──────────────────────┬──────────────────────┐
│ Receptor Target │ Biological Action │ Clinical Benefit │
├──────────────────────┼──────────────────────┼──────────────────────┤
│ Alpha-1 Receptors │ Potent systemic │ Reverses hypotension,│
│ │ vasoconstriction │ reduces airway edema │
├──────────────────────┼──────────────────────┼──────────────────────┤
│ Beta-1 Receptors │ Increases cardiac │ Restores depressed │
│ │ inotropy & chronotropy cardiac output │
├──────────────────────┼──────────────────────┼──────────────────────┤
│ Beta-2 Receptors │ Bronchial smooth │ Halts wheezing and │
│ │ muscle relaxation │ relieves air hunger │
├──────────────────────┼──────────────────────┼──────────────────────┤
│ Mast Cell Inhibition │ Suppresses mediator │ Halts release of │
│ │ degranulation │ histamine & leukotrien│
└──────────────────────┴──────────────────────┴──────────────────────┘
Why antihistamines and steroids cannot save lives¶
Bystanders and untrained individuals frequently make the fatal mistake of administering oral diphenhydramine (Benadryl) or oral steroids during anaphylaxis: - Antihistamines: H1-receptor antagonists merely block peripheral skin receptors to relieve itching and reduce hives. Antihistamines have zero biological ability to reverse life-threatening laryngeal edema, cannot relax bronchial spasms, and cannot raise collapsing blood pressure. - Corticosteroids: Steroids alter cellular transcription and require 4 to 6 hours to demonstrate any clinical anti-inflammatory effect. - The fatal delay: In fatal cases of anaphylaxis, the single most common clinical finding is the delayed administration of epinephrine while caregivers waited for oral antihistamines to work. Epinephrine must always be administered first.
Why intramuscular thigh injection is mandatory¶
Epinephrine must be injected intramuscularly (IM) into the anterolateral aspect of the middle third of the thigh (the vastus lateralis muscle): - Rapid absorption: The vastus lateralis muscle possesses a vast vascular network. Pharmacokinetic trials demonstrate that an intramuscular thigh injection achieves therapeutic peak plasma concentrations in the bloodstream within 8 minutes. - Why the arm fails: Injecting epinephrine into the deltoid muscle of the arm or into subcutaneous fat requires over 34 minutes to achieve equivalent plasma levels, a delay that is frequently fatal during progressive respiratory collapse.
Auto-injector deployment protocols¶
Epinephrine auto-injectors (such as EpiPen, Auvi-Q, Jext, Emerade, and generic equivalents) are spring-loaded medical syringes designed for rapid civilian deployment through clothing:
GENERIC AUTO-INJECTOR DEPLOYMENT
[ 1. GRASP ] ──► Form fist around device; never touch ends
│
[ 2. PULL ] ──► Pull off blue safety release cap vertically
│
[ 3. SWING ] ──► Position orange tip against outer mid-thigh
│
[ 4. PUSH ] ──► Push firmly until spring-loaded needle triggers
│
[ 5. HOLD ] ──► Hold firmly against thigh for 2 to 3 seconds
│
[ 6. MASS ] ──► Remove device; rub injection site for 10 sec
- Form a secure fist: Wrap your fingers around the center barrel of the auto-injector. Never place your thumb, fingers, or palm over either end of the device to prevent accidental self-injection into the hand.
- Remove safety release: Pull the safety release cap (often blue or yellow) straight off the back of the device.
- Position against the outer thigh: Aim the needle end (often orange or black) toward the outer middle portion of the thigh. The injection can be delivered directly through jeans, trousers, or athletic leggings. Avoid thick seams, metal zippers, or contents in pockets.
- Execute firm contact: Push the device firmly into the thigh at a 90-degree angle until you hear and feel a loud click, indicating the spring has fired the needle into the muscle.
- Hold in place: Maintain continuous, firm pressure against the thigh for 2 to 3 seconds (check specific manufacturer instructions on newer devices; older units required 10 seconds).
- Massage the site: Remove the unit, note the exact time of delivery, and gently massage the injection site for 10 seconds to enhance vascular uptake.
Administering a second dose¶
Between 10 and 20 percent of individuals suffering from severe anaphylaxis require a second dose of epinephrine: - Indications for dose two: If the patient symptoms fail to resolve, if breathing difficulties worsen, or if the individual experiences progressive lightheadedness 5 to 15 minutes after the first injection, administer a second fresh auto-injector into the opposite thigh.
The fatal postural collapse: empty vena cava syndrome¶
How a patient is physically positioned during anaphylactic shock can dictate whether they survive:
THE EMPTY VENA CAVA COLLAPSE CASCADE
Systemic vasodilation and massive capillary leakage
│
▼
Up to 35% of intravascular blood volume leaks into tissues
│
▼
Patient sits up or stands on feet abruptly
│
▼
GRAVITY POOLS REMAINING BLOOD IN LEGS AND SPLANCHNIC BEDS
│
▼
Venous return to the right atrium drops to ZERO
│
▼
INSTANT PULSELESS ELECTRICAL ARREST AND CARDIAC COLLAPSE
The danger of upright positioning¶
During anaphylaxis, systemic histamine and leukotriene release causes profound peripheral vasodilation and increased vascular permeability. Within 10 minutes, up to 35 percent of the patient total circulating blood volume can leak out of blood vessels into interstitial spaces. - The empty ventricle phenomenon: If a person in this hypovolemic state suddenly stands up, walks, or is lifted upright, gravity immediately drains the remaining blood into the lower limbs. Blood return through the inferior vena cava to the right side of the heart ceases completely. The heart pumps completely empty, resulting in instantaneous pulseless electrical activity (PEA) or asystole. - Positioning rules: - Recumbent posture: Keep the patient lying flat on their back (supine) with their legs elevated 30 centimeters (12 inches) to facilitate venous return. - Respiratory distress exception: If the patient is struggling severely to breathe, allow them to sit in a semi-reclined position, but never let them stand or walk. - Recovery position: If the patient is dizzy, vomiting, or loses consciousness, roll them onto their side in the recovery position to protect the airway while maintaining a flat horizontal posture.
The biphasic reaction threat¶
A critical clinical characteristic of anaphylaxis is the biphasic reaction:
- The phenomenon: A biphasic reaction is the spontaneous recurrence of severe, life-threatening anaphylactic symptoms hours after the initial reaction has completely resolved, with zero additional allergen exposure.
- Occurrence rate: Biphasic reactions occur in approximately 5 to 15 percent of anaphylactic events, typically appearing between 4 and 12 hours after the primary reaction.
- Mandatory hospital observation: Because biphasic reactions cannot be predicted based on the severity of the initial presentation, every individual who experiences anaphylaxis must be transported by emergency ambulance to a hospital emergency department for a minimum 4 to 8 hour clinical observation period.
Prevention and personal allergy preparedness¶
- Carry two auto-injectors at all times: Never leave home without an auto-injector; because second doses are frequently required, always carry devices in pairs.
- Protect devices from temperature extremes: Never store epinephrine auto-injectors in car glove boxes, under direct sunlight, or in refrigerators; extreme heat degrades epinephrine rapidly, while freezing disables the mechanical firing spring.
- Inspect the medication window monthly: Ensure the epinephrine liquid inside the viewing window is clear and colorless; discard and replace the unit if the fluid appears cloudy, pinkish, or contains sediment.
- Establish a written Allergy Action Plan: Coordinate with an allergist to document personal triggers, early warning signs, and emergency contact protocols.
- Wear medical alert identification: Wear an engraved medical alert bracelet or configure mobile emergency health cards detailing severe food or venom allergies.
First aid certification
Hands-on certification through the American Red Cross, St John Ambulance, or national first aid bodies allows practical practice with trainer auto-injectors, emergency positioning, and resuscitation drills.
Frequently asked questions¶
Why should I give epinephrine before giving an antihistamine like Benadryl?¶
Epinephrine is the only medication that reverses the life-threatening physiological mechanisms of anaphylaxis. It constricts blood vessels to raise blood pressure, relaxes lung muscles to open airways, and reduces throat swelling. Antihistamines only block histamine receptors in the skin to reduce itching and hives; they cannot stop throat closure, breathing failure, or cardiovascular shock. Waiting for an antihistamine to work before using epinephrine is the primary cause of fatal anaphylaxis.
Can an epinephrine auto-injector be injected through heavy clothing?¶
Yes. Epinephrine auto-injectors are engineered with powerful internal spring mechanisms that fire the needle through heavy clothing, including thick denim jeans and fleece trousers. Aim for the outer middle of the thigh and avoid thick fabric seams, buttons, zippers, or hard objects inside pockets.
Why is it dangerous for someone having a severe allergic reaction to stand up?¶
During anaphylaxis, blood vessels dilate massively and blood fluids leak out of capillaries, drastically dropping internal blood volume. If the person stands up or walks, gravity forces the remaining blood to pool in their legs, completely cutting off blood return to the heart. This triggers sudden, fatal cardiac arrest known as empty vena cava syndrome. Always keep the person lying flat with their legs elevated.
What should I do if the person feels better after one injection?¶
You must still call emergency services and transport the person to an emergency hospital. The effects of epinephrine begin to wear off after 15 to 20 minutes, and the allergic reaction can return. Furthermore, up to 15 percent of patients experience a biphasic reaction, where severe breathing difficulty and shock recur 4 to 12 hours later without any secondary contact with the allergen.
Where exactly on the body should an epinephrine injection be given?¶
Epinephrine must always be injected into the anterolateral (outer) middle third of the thigh muscle. The large thigh muscle has an exceptionally rich vascular supply that absorbs epinephrine into the bloodstream within 8 minutes. Injecting epinephrine into the arm or buttocks results in significantly slower absorption and should never be done.