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Spotting the Signs of a Serious Allergic Reaction

Medically reviewed by Mario Quintanilla, MD — Emergency Medicine Physician, Medical Director, Bellaire ER

Content reviewed and edited by Dr. Mario Quintanilla, MD. Walk in or call (713) 660-0001 anytime.

Illustration for Spotting the Signs of a Serious Allergic Reaction

A serious allergic reaction (anaphylaxis) usually starts within minutes to two hours of exposure to a trigger. The key warning signs are trouble breathing, swelling of the lips, tongue, or throat, widespread hives, repeated vomiting, and dizziness or fainting. A reaction that involves two or more body systems — for example, hives plus vomiting — should be treated as anaphylaxis. If you see these signs, use an epinephrine auto-injector immediately if one is available, then call 911. Anaphylaxis can turn life-threatening in minutes, and epinephrine works best when given early.

What causes serious allergic reactions?

An allergic reaction happens when the immune system overreacts to a normally harmless substance and releases chemicals such as histamine. The triggers most likely to cause severe reactions are:

  • Foods — peanuts, tree nuts, shellfish, fish, milk, eggs, soy, wheat, and sesame
  • Insect stings — bees, wasps, hornets, and fire ants
  • Medications — most often antibiotics (such as penicillin) and NSAIDs
  • Latex — gloves, balloons, and some medical supplies

Inhaled allergens like pollen, dust mites, and pet dander usually cause milder symptoms (sneezing, itchy eyes, congestion), but they can trigger serious breathing trouble in people with asthma.

Illustration for Spotting the Signs of a Serious Allergic Reaction

Red flags — go to the ER now

Call 911 or come straight to the ER if any of the following occur after a known or suspected exposure:

  • Trouble breathing — wheezing, noisy breathing, persistent cough, or shortness of breath
  • Throat or mouth swelling — tight throat, hoarse voice, trouble swallowing, or drooling
  • Circulation problems — dizziness, fainting, pale or clammy skin, or a rapid, weak pulse (signs of dangerously low blood pressure)
  • Two or more body systems involved — for example, hives plus vomiting, or facial swelling plus abdominal pain and cramping
  • Widespread hives with any other symptom, or a feeling of dread or confusion
  • Any reaction after a trigger that previously caused a severe reaction, even if symptoms seem mild at first

If epinephrine is available, use it first, then call 911. Do not wait to see if symptoms improve on their own. Lay the person flat with legs raised (or sitting up if breathing is hard), and give a second dose of epinephrine if there is no improvement after about 5 minutes. Anyone who receives epinephrine still needs emergency evaluation — symptoms can return hours later in a second wave (a biphasic reaction).

Medical illustration 3 - Spotting the Signs of a Serious Allergic Reaction

What about milder symptoms?

A few localized hives, mild itching, or a runny nose without breathing, swallowing, or circulation problems can often be managed with an oral antihistamine and close observation. Watch the person for at least an hour, because mild symptoms can progress. Antihistamines are never a substitute for epinephrine in a severe reaction.

ER, urgent care, or telemedicine?

  • ER (or 911): any of the red flags above — breathing trouble, throat swelling, fainting, or symptoms in more than one body system. Our guide on when to visit the ER covers other emergency warning signs.
  • Urgent care or a doctor’s visit: a mild rash or localized hives with no breathing or swallowing symptoms and no history of severe reactions.
  • Telemedicine: questions about a mild rash, medication side effects, or whether a past reaction needs allergy follow-up. You can book a telemedicine visit with our physicians.

When in doubt, choose the ER. Our allergic reaction emergency care team at Bellaire ER treats reactions 24/7 with epinephrine, IV medications, airway management, and on-site monitoring for biphasic reactions.

Medical illustration 4 - Spotting the Signs of a Serious Allergic Reaction

Sources

If you or a loved one is having a serious allergic reaction, call 911 or come directly to Bellaire ER at 5302 Bellaire Blvd, Bellaire, TX 77401 — open 24/7. Call us at (713) 660-0001.