Patient Basics: Anaphylaxis

Anaphylaxis is a severe allergic reaction that affects the whole body. It starts within minutes to a couple of hours after contact with a trigger such as a food, an insect sting or a medicine, and it can close the throat and drop blood pressure. It is an emergency. Use epinephrine right away and call 911.

Key points

• Epinephrine is the only medicine that stops anaphylaxis. Antihistamines such as diphenhydramine (Benadryl) are not enough.
• Signs often involve two or more body parts at once, such as hives plus trouble breathing, or swelling plus vomiting.
• Give epinephrine early, even if you aren’t sure. Waiting is riskier.
• Always go to an emergency department afterward, because symptoms can return hours later.
• Anyone who has had anaphylaxis should carry two epinephrine devices and see an allergist.

What causes anaphylaxis

In an allergy, the immune system treats something harmless as a threat. Immune cells called mast cells release histamine and other chemicals all at once. These make blood vessels leak and widen, airways tighten, and skin swell and itch.

Common triggers:

• Foods: peanuts, tree nuts, shellfish, fish, milk, eggs, sesame, wheat and soy
• Insect stings: bees, wasps, hornets, yellow jackets and fire ants
• Medicines: penicillin and other antibiotics, aspirin and ibuprofen, X-ray dye, and some drugs used during surgery
• Latex in gloves and medical supplies
• Red meat in people with alpha-gal syndrome, which follows some tick bites. Reactions often start 2 to 6 hours after eating.
• Exercise, sometimes only after eating a certain food

In some people, no trigger is ever found. Doctors call this idiopathic anaphylaxis.

Symptoms of anaphylaxis

• Skin: hives, itching, flushing, or swelling of the lips, tongue, eyes or face
• Breathing: throat tightness, a hoarse voice, trouble swallowing, coughing, wheezing or shortness of breath
• Heart and blood pressure: dizziness, a fast or weak pulse, pale or bluish skin, fainting
• Stomach: vomiting, cramps or diarrhea
• Other: a sense that something terrible is about to happen, confusion, or anxiety

Anaphylactic shock is anaphylaxis with a dangerous fall in blood pressure. A person may feel faint, become confused or pass out. Some people have no skin symptoms at all, so don’t rule out anaphylaxis because there are no hives.

Mild allergy or anaphylaxis?

A mild allergic reaction stays in one place and doesn’t affect breathing or blood pressure. It might be sneezing, a runny nose and itchy, watery eyes, or a few itchy hives or a red, itchy patch of skin. A non-drowsy antihistamine, a cool compress and avoiding the trigger usually take care of it.

Treat it as anaphylaxis if there is any trouble breathing or swallowing, swelling of the tongue or throat, repeated vomiting, dizziness or fainting, or if skin symptoms spread quickly after a known trigger.

What to do in an emergency

1. Give epinephrine right away. Use an auto-injector in the outer thigh, through clothing if needed, or an epinephrine nasal spray if that is what was prescribed.
2. Call 911, even if the person starts to feel better.
3. Lie the person flat with legs raised. If breathing is hard, let them sit up. If they are vomiting, turn them on their side. Don’t let them stand or walk suddenly.
4. Give a second dose after 5 to 15 minutes if symptoms don’t improve or come back.
5. Start CPR if the person stops breathing or has no pulse.

In the emergency department, the team may give more epinephrine, oxygen, IV fluids and breathing treatments. Most people are watched for several hours, because a second wave of symptoms (a biphasic reaction) can happen without new contact with the trigger.

After anaphylaxis

• See an allergist. Skin and blood tests can confirm the trigger. A blood test called tryptase, ideally taken within a few hours of the reaction, can help confirm it was anaphylaxis.
• Carry two epinephrine devices at all times, and check expiration dates.
• Get a written action plan and share it with family, school or work.
• Wear a medical ID bracelet or keep the information on your phone.
• Read food labels and tell restaurant staff about your allergy every time.
• Ask about treatments that lower your risk. Allergy shots for insect venom work very well. Oral immunotherapy can raise the amount of peanut a person can tolerate. Omalizumab, an injected medicine, can lower the chance of a reaction to accidental bites of trigger foods.
• Review your medicines. Beta-blockers for blood pressure or the heart can make anaphylaxis harder to treat.

Common questions about anaphylaxis

What is the best medicine for hives and itching?

For hives without other symptoms, a non-drowsy antihistamine such as cetirizine, loratadine or fexofenadine works well and lasts all day. Diphenhydramine works but causes sleepiness and wears off sooner. Hives with throat swelling, trouble breathing or dizziness need epinephrine and 911, not antihistamines alone.

What if hives don’t go away with Benadryl?

Hives that last days or keep returning need a doctor visit. Your doctor may suggest a daily non-drowsy antihistamine, sometimes at a higher dose than the label. Hives lasting more than 6 weeks are called chronic hives and can be treated with injected medicines such as omalizumab or dupilumab.

Are there home remedies for an allergic reaction?

For mild reactions, a cool compress, an oatmeal bath, hydrocortisone cream on itchy skin and an antihistamine can help. No home remedy treats anaphylaxis. If someone has trouble breathing, swelling in the mouth or throat, or feels faint, use epinephrine and call 911.

Can anaphylaxis keep coming back?

Yes. Repeated contact with the same trigger can cause it again, and reactions can be just as bad or worse. Some people have repeated anaphylaxis with no clear trigger, sometimes from a mast cell disorder. An allergist can test for these and may suggest daily medicines to prevent attacks.

Questions to ask your doctor

• What caused my reaction, and how can I avoid it?
• Which epinephrine device should I carry, and how do I use it?
• Should I see an allergist for testing or desensitization?
• Do any of my medicines make anaphylaxis harder to treat?
• What should my written action plan say?

Learn more

• Anaphylaxis (MedlinePlus, U.S. National Library of Medicine)
• Anaphylaxis (NHS)

This guide is general information and doesn’t replace advice from your own doctor. Last updated October 2026.