Allergic Reactions
An allergic reaction occurs when the immune system responds to a substance such as food, medicine, an insect sting, or latex. Reactions can be mild or life-threatening.
About allergic reactions
An allergic reaction occurs when the immune system responds to a substance such as food, medicine, an insect sting, or latex. Reactions can be mild or life-threatening.
What can cause it?
Foods, medicines, insect stings, latex, animals, and environmental allergens are common triggers.
A reaction may involve the skin, breathing, circulation, or digestive system and can worsen quickly.
What a patient may notice
Symptoms vary from person to person. Possible signs may include:
- Hives, itching, or swelling
- Runny nose, cough, or wheezing
- Nausea, vomiting, dizziness, or throat tightness
How clinicians find the cause
Diagnosis begins with the timing of symptoms and exposures. A clinician may review photographs, ingredient labels, medicines, and prior reactions. Allergy skin or blood testing may be arranged after the acute episode when appropriate.
Treatment and next steps
- 1
Anaphylaxis requires immediate epinephrine and 911 activation.
- 2
Milder reactions may be treated with clinician-recommended antihistamines or other medicines, but these do not replace epinephrine for anaphylaxis.
- 3
Observation may be needed because severe symptoms can recur.
Supporting recovery
- Stop exposure when it can be done safely.
- Use an epinephrine auto-injector exactly as prescribed for severe symptoms, then call 911.
- Do not rely on an antihistamine alone for breathing, throat, fainting, or multi-system symptoms.
What to expect
Most mild allergic reactions improve after the trigger is removed and appropriate treatment is given. A future exposure can cause a similar, milder, or more severe reaction, so the previous reaction does not reliably predict the next one. People with a significant reaction benefit from identifying the trigger and having a written emergency plan.
Possible complications
- Anaphylaxis with airway swelling, breathing difficulty, or dangerously low blood pressure
- A second wave of symptoms after the initial reaction appears to improve
- Injury from fainting or complications caused by delayed emergency treatment
When to Contact a Medical Professional
A same-day medical evaluation is recommended, especially when symptoms are severe, worsening, or accompanied by fever, vomiting, weakness, or significant pain.
Call 911 for trouble breathing, throat or tongue swelling, fainting, or symptoms affecting more than one body system.Get emergency help for severe trouble breathing, new chest pressure, fainting, seizure, sudden weakness on one side, uncontrolled bleeding, blue or gray lips, or a person who is difficult to wake.
Ways to lower risk
- Identify and avoid confirmed triggers.
- Read labels, tell healthcare professionals about allergies, and wear medical identification when recommended.
- Keep prescribed epinephrine available and know how to use it.
Trusted health information
This CHS patient-education page uses original wording and links to MedlinePlus for additional information.