person breaking out in hives for no apparent reasonShare on Pinterest
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Sometimes the source of the raised, red, and itchy bumps on your skin can be a mystery.

One reason hives can be so surprising is that they can be caused by many things you might not expect including stress and exercise. Most of these unexpected causes for hives aren’t serious, but some are a sign it’s a good idea to make a medical appointment.

Read on for more information about reasons you may have hives.

Hives are an itchy reaction on your skin. They happen when a chemical called histamine is released in your body.

They can appear anywhere on your body and can be tiny pinprick-sized bumps or large raised areas that cover an entire limb. Hives often appear red or pink on white or light skin. People with darker skin might have hives that are slightly lighter or slightly darker than the skin surrounding them.

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Hives on dark skin.
Created by Samuel Freire da Silva, M.D. in homage to The Master And Professor Delso Bringel Calheiros. Image obtained from Dermatology Atlas

No matter the color of your hives rash, all hives share these qualities:

  • raised
  • itchy
  • tender
  • round, oval, or uneven shaped
  • clearly defined border

Hives are also called urticaria. Sometimes, the cause of hives is obvious. For instance, you might have an immediate reaction to something you’re allergic to such as pollen or pet dander. However, the cause isn’t always clear. Hives can be sudden and surprising and seem to not have a cause.

Hives are a very common skin reaction that can come from some unexpected sources. Some causes you might not have thought of include:

  • Colds and other viral infections. Sometimes hives can be caused by your immune system fighting colds and other viral infections. These hives often appear near the end of your cold or virus as you’re starting to feel better. They’re most common in children but can happen to anyone.
  • Bacterial infections. Bacterial infections such as strep throat or urinary tract infections can also sometimes trigger hives as your body reacts to the bacteria. These hives will fade as the antibiotics help your body fight the infection. They might peel before healing completely.
  • Chronic conditions such as lupus. Hives that last for longer than 6 weeks might be a sign of an autoimmune condition such as lupus, type 1 diabetes, thyroid disease, or rheumatoid arthritis. These hives won’t go away on their own. It’s a good idea to make a medical appointment to get this type of hive checked out and see if a chronic condition is the cause.
  • Stress. Stress can raise your internal body temperature and release adrenalin and other chemicals that might trigger hives. Stress hives tend to be located on the face, neck, chest, and arms. They’re common in people with eczema, allergies, or sensitive skin.
  • Temperature changes. Suddenly encountering hot or cold, such as stepping into a steamy shower or entering a swimming pool, can cause histamine to release and hives to form. Hives that form in response to temperature are called cold urticaria. A red and itchy area of skin often forms around these hives.
  • Tight clothing. Tight clothing can cause friction that leads to irritation and to hives. Clothes that sit close to your skin can also push any bacteria on your skin surface into your pores and hair follicles.
  • Exercise. A chemical called acetylcholine is released in your body when you exercise can affect your skin cells and cause irritation and hives. Exercise hives are known as exercise-induced urticaria. Some people have additional symptoms along with hives such as shortness of breath, headache, flushing, and stomach cramps.
  • Inflammation of blood vessels. An inflammation of your blood vessels known as vasculitis can cause painful hives. They can leave a bruise on your skin and last for several days. Vasculitis is a serious medical condition that requires treatment by a medical professional.
  • Medications. Some medications, including nonsteroidal anti-inflammatory drugs (NSAIDs), antibiotics, and opioids can cause an allergic reaction that leads to hives. Hives following medication might be the first sign of a medical emergency called anaphylaxis. Other symptoms of anaphylaxis include shortness of breath, wheezing, vomiting, and loss of consciousness.

Hives are often caused by identifiable allergies. Avoiding these allergens can help you avoid breaking out in hives. Common hive triggers include:

Hives can be a sign of a serious allergic reaction that needs emergency medical emergency attention. It’s important to take action if you hives along with any of these symptoms:

  • wheezing
  • tightness in your chest or throat
  • trouble breathing
  • trouble talking
  • swelling in your throat, face, or tongue
  • nausea
  • lightheadedness

If you are experiencing any of these symptoms, call 911.

A doctor may prescribe you an epinephrine injector, such as an EpiPen, for future allergic reactions. They’ll teach you how to use it and answer any questions you might have. You’ll keep your epinephrine injector on hand and use it if you develop hives in the future.

You’ll still need to visit the emergency room after you use your epinephrine injector, but it can stop anaphylaxis from becoming deadly.

A doctor can diagnose hives and help you find the cause. They might recommend you keep a food diary to find out if there is any link between food and your hives.

You might be sent to an allergist, a doctor who specializes in treating allergies, for additional testing. This might include blood work and urine tests to look for chemicals in your body that might tell the allergists what’s causing your hives.

You might also have a skin biopsy, especially if your allergist suspects vasculitis causing your hives. Hives that have lasted for longer than 6 weeks will likely necessitate testing for underlying chronic conditions.

Sometimes, a specific cause isn’t found. In this case, your hives will be diagnosed as idiopathic urticaria. The word “idiopathic” means unknown. In this case, your doctor will still be able to help you with a treatment plan, but you won’t be able to tell what to avoid to prevent hives in the future.

Treatment for hives will depend on the severity of your hives and on the cause. For instance, you’ll need to avoid the cause of your hives if it has been found.

Your doctor will work with you to find the right treatment for you. Common options include:

  • Antihistamines. Both over-the-counter and prescription antihistamines block histamine and can treat hives. You might find that certain antihistamines are more effective for you than others. Sometimes a combination of antihistamines is recommended.
  • Anti-itch lotions. Lotions to calm down itching and redness can provide relief from hives and prevent scratching.
  • Antibiotics. Hives that are linked to bacterial infections can be treated with antibiotics
  • Corticosteroids. Corticosteroids can be taken on a short-term basis to help with severe cases of hives.

Read more about hives treatment options.

Hives can sometimes be surprising and have no obvious cause. There are actually a wide variety of things that can cause hives, including stress, colds, exercise, and temperature changes. Chronic hives can point to an underlying condition such as lupus.

An allergist can help you determine the cause of your hives and start treatment.