Allergic Reaction: Signs, Severity and What to Do Now

What an allergic reaction looks like

Symptoms depend on where the allergen enters and how sensitive you are. They usually begin within minutes of eating, being stung or injected, and almost always within 2 hours.

Mild to moderate reactions

• Skin: hives (raised, itchy welts that move around), flushing, itching, an eczema flare
• Nose and eyes: sneezing, runny or blocked nose, red, itchy, watery eyes
• Mouth: tingling or itching of the lips, tongue or palate (common with raw fruit in pollen-allergic people)
• Gut: mild nausea or stomach cramps
• Local swelling: puffy lips or eyelids (angioedema) without breathing problems

Severe reaction: anaphylaxis

Anaphylaxis is likely when skin or mouth symptoms combine with any of these, or when any of these follow a known allergen:

• Airway: throat tightness, swollen tongue, hoarse voice, difficulty swallowing
• Breathing: wheezing, shortness of breath, persistent cough, noisy breathing
• Circulation: dizziness, fainting, pale clammy skin, rapid weak pulse, collapse
• Gut: repeated vomiting, severe belly pain (especially after stings or medicines)
• A sense of impending doom

Skin symptoms are absent in up to 1 in 5 cases of anaphylaxis, so do not wait for hives.

What to do right now

If you suspect anaphylaxis

1. Give epinephrine into the outer thigh using an auto-injector (through clothing is fine).
2. Call 911 or your local emergency number. Say "anaphylaxis".
3. Lie flat with legs raised. If breathing is hard, sit up. If pregnant, lie on the left side. Do not stand or walk suddenly.
4. Give a second dose after 5 to 15 minutes if there is no improvement and a second injector is available.
5. Go to hospital even if you recover. Symptoms can return hours later (a biphasic reaction).

Antihistamines do not treat anaphylaxis. Give epinephrine first.

For a mild reaction

• Stop or move away from the trigger: remove a bee stinger by scraping, wash skin after contact.
• Take a non-drowsy antihistamine such as cetirizine or loratadine, following the label dose.
• Use a cool compress for itchy or swollen skin.
• Watch closely for 2 to 4 hours for signs of a more severe reaction.

Common triggers

Allergic reactions in babies after vaccines

Most reactions after infant vaccines are normal immune responses, not allergies: fussiness, a low fever, and redness or swelling at the injection site for 1 to 2 days. Spitting up is common in healthy babies and, on its own, is not a sign of a vaccine allergy.

True allergic reactions to vaccines are rare, about 1 per million doses, and occur within minutes to 4 hours, which is why clinics observe babies for 15 minutes afterward. Seek emergency help if a baby has hives with facial swelling, breathing difficulty, repeated forceful vomiting, or becomes pale and floppy after a vaccine. Call your pediatrician for high fever, inconsolable crying for more than 3 hours, or a large area of swelling.

Things that look like allergic reactions

• Viral rashes and viral hives in children, which often appear during or after a cold
• Irritant reactions to soaps or plants, without immune involvement
• Food intolerance (lactose, for example), which causes bloating and diarrhea but not hives or breathing problems
• Panic attacks, which cause throat tightness and breathlessness without swelling or rash
• Flushing from alcohol, spicy food or niacin

After the reaction: getting a diagnosis

Anyone who has had a significant reaction should see a doctor or allergist, who will take a detailed history and may arrange skin-prick tests, blood tests for specific IgE antibodies, or a supervised challenge. If you are at risk of anaphylaxis you will be prescribed two epinephrine auto-injectors and an allergy action plan. Our page on allergic reaction as a condition explains how allergies develop and are treated long term.

Frequently asked questions

What are the first signs of an allergic reaction?

Early signs include itching or tingling in the mouth, hives, flushing, sneezing, a runny nose or watery eyes. In severe reactions, throat tightness, coughing, wheezing, swelling of the lips or tongue, vomiting or dizziness follow quickly. These usually start within minutes to 2 hours of exposure.

How long does an allergic reaction last?

Mild reactions usually settle within a few hours, especially with an antihistamine. Hives can come and go for a day or two. After anaphylaxis, symptoms can return 4 to 12 hours later even without re-exposure, which is why people are observed in hospital.

Is spitting up a sign of an allergic reaction to infant shots?

Not by itself. Spitting up is very common in healthy babies, and vaccines can make babies fussy for a day or two. Signs of a true allergic reaction include hives, facial swelling, breathing difficulty, repeated forceful vomiting or becoming floppy within minutes to hours. Those need emergency care.

When should I use an EpiPen?

Use epinephrine at the first signs of anaphylaxis: breathing difficulty, throat or tongue swelling, faintness, or hives with vomiting after a known allergen. When in doubt, give it. It is safe for most people, and delays are linked to worse outcomes. Then call 911 or your local emergency number.

Can you suddenly become allergic to something?

Yes. Allergies can develop at any age, including to foods or medicines you previously tolerated. Adults commonly develop new shellfish, tree nut or medicine allergies. Any new reaction should be assessed so you know what to avoid and whether you need an epinephrine auto-injector.