What Causes Hives?

Hives are welts on the skin that appear in varying sizes, shapes, and patterns, and can have numerous causes. Anybody can get them, and they can appear out of nowhere. One day you’re hive-free, and the next, you’re not.

How do you know the rash is hives? No matter what’s happening inside your body, hives look the same. They’re usually red or skin-colored bumps that can blanch (turn white) when you press on their center. They can change shapes and move around, and may break out all over your body, says Adam Friedman, MD, a professor and the chair of dermatology at the George Washington University School of Medicine and Health Sciences in Washington, DC.

In addition, if the rash comes and goes within 24 hours, it is most likely hives. But hives can either be acute or chronic. The two types share many common triggers. The big difference is timing. For acute hives, you’re just dealing with one bout of hives, or they may come and go, but they resolve within six weeks.

If you have chronic hives, the rashes continue to appear and disappear for longer than six weeks. In these cases it may be tougher to pinpoint a cause.

Here’s more on common causes and what happens in the body that triggers hives.

What Causes Hives?

Experts know a lot more about what’s happening in the body when hives show up versus what actually triggers them and why.

“While we can’t always identify what’s causing a hive, we do know what happens in the body to produce the hive,” says Dr. Friedman.

To understand the basics, it helps to know a little bit about the body’s allergic response. Your body produces mast cells, which act as part of your allergic response. When your body perceives it’s been exposed to a threat, the mast cells may secrete numerous chemicals, most notably histamine, says Sarina Elmariah, MD, PhD, an assistant dermatologist and the director of the Itch Clinic at Mass General Brigham in Boston.

In most situations, histamine causes your blood vessels to dilate and leak fluid, creating the raised areas on your skin (hives). Histamine also affects your nerve cells, signaling your skin to itch, Dr. Elmariah says.

A lot of people assume that hives are the result of the release of histamine, a chemical your body produces in response to allergens. Histamine does play a role in the majority of cases, but it’s important to know it can be more complex than that, Elmariah says. “Some hives are beyond the scope of histamine and involve other chemicals,” she says.

In other situations, basophils, another type of white blood cell, may be activated to release their hive-producing chemicals and cause the same effect, Elmariah says.

If you’re allergic to things like insects, medications, latex, or certain foods and you come in contact with your allergen, a protein found throughout the blood and body tissues called immunoglobulin E (IgE) will bind the allergen and signal the mast cells to release their contents, histamine included, Elmariah says.

Hives may also result when other blood protein antibodies (besides IgE) bind directly onto mast cells, prompting them to release their contents, or in other situations, mast cells may be completely degranulated or destabilized, which induces the chemical release, Elmariah says.

Forty to 50 percent of hives cases are associated with angioedema, which involves swelling of the eyes, mouth, hands, feet, or throat.

It can be caused by medications, allergic reactions, or a hereditary deficiency of some enzymes in your body. Meanwhile, anaphylaxis can result if you’ve been exposed to something you’re allergic to, especially if that allergy is severe.

Common Triggers of Hives

A more practical question about why hives show up may be: What are the factors that trigger the above reactions in your body? The list of possible suspects is long, and does not differ greatly for acute versus chronic hives. “Some people with chronic hives can be exposed to the same acute trigger over and over again,” Elmariah says.

Some of the most common causes of hives are food, drugs, and infections.

Foods That Trigger Hives

Hives from food are usually related to food allergies, including peanuts, eggs, milk, fish, shellfish, and other kinds of nuts.

You may even have hives as a result of a pseudoallergic reaction, a reaction to a food or chemical that mimics the symptoms of an allergic reaction, but without IgE antibodies being produced against the offending item.

Pseudoallergic reaction triggers can include artificial food dyes, tomatoes, preservatives, sweeteners, herbs, wine, high dietary fats, alcohol, orange oil, strawberries, and some yellow and red food dyes.

Drugs That Trigger Hives

On the drug front, there are several that can cause hives, including nonsteroidal anti-inflammatory drugs (NSAIDs), aspirin, muscle relaxants, antibiotics (especially penicillin and sulfa), diuretics, IV radiocontrast, angiotensin converting enzyme (ACE) inhibitors, and angiotensin receptor blockers, says Anthony M. Rossi, MD, a dermatologist and Mohs surgeon at Memorial Sloan Kettering Cancer Center in New York City.

Infections That Trigger Hives

Infections might include viral ones like the common cold and hepatitis or bacterial ones like strep throat or urinary tract infections, Dr. Rossi says.

Other Things That Trigger Hives

You can also get hives from touching things you may be allergic to. For instance, if you’re allergic to latex, you might get a hive if you touch a balloon or latex glove. The same is the case if you’re allergic to dogs and you’re exposed to dander.

Other things that can cause hives include blood transfusions, insect bites or stings, plants, and to a smaller degree, stress (usually it takes emotional stress combined with some other factor to trigger hives, rather than emotional stress triggering hives on its own).

Environmental stimuli may cause hives, something experts call inducible urticaria (often referred to as physical urticaria). Things like pressure on your skin, exercise, cold, heat, water, sun exposure, and vibrations can instigate an episode of hives.

“You might simply be walking by a construction site or riding a train, and because of the vibrations from these things, you break out,” Elmariah says. These types of hives occur less frequently than those caused by food, drugs, or infections.

Chronic hives, meanwhile, can be caused by any of the above, but they may also be a sign of a health issue or autoimmune disease. “Autoimmune diseases in general have been strongly associated with the development of chronic hives,” Elmariah says.

Yet some have a stronger link to hives, such as thyroid disease, type 1 diabetes, celiac disease, lupus, and rheumatoid arthritis.

Are Hives Contagious?

Fortunately, although hives can be extremely itchy, they’re not contagious, and an individual hive usually disappears within 24 hours, Elmariah says. (If a single hive lasts for longer than 24 hours, the cause could be more serious and you should call your doctor.)

For some people, hives can be so chronic in nature (with single hives coming and disappearing continuously) that they can be an issue for months.

Hives can even be so problematic that they interrupt healthy sleep, work, and school and isolate people socially, affecting mental as well as physical health, Elmariah says.

When Should I See My Doctor About Hives?

Because hives usually disappear within 24 hours, it’s probably not necessary to see your doctor if you have just one bout (and they go away within a one-day time frame). If hives do not go away on their own in a few days (or if a single hive does not go away within 24 hours), do see your doctor.

If your throat starts to swell or you’re having trouble breathing, seek emergency care immediately.

If hives appear to come and go for longer than six weeks, they’re considered chronic, and it’s best to visit a dermatologist or allergist to see what’s going on, Rossi says.

The caveat? “Although hives in and of themselves generally aren’t life-threatening, they can be associated with a condition called angioedema and a severe allergic reaction known as anaphylaxis, both of which can cause breathing issues and require emergency care,” Elmariah says.

Emergency visits aside, when you do see your doctor to determine what’s causing your hives, come prepared with information. Here’s what Elmariah recommends having on hand:

Any trigger you’ve identified, including foods, chemicals, medication exposures, even travel history
A list of all your current or recent medications
Your medical history, including other personal and family history of medical problems
A list of medications (and doses) you’ve already tried and the effects they’ve had

Common Questions & Answers

What are hives?

Hives are welts that appear on the skin in various sizes, shapes, and patterns. They can occur suddenly and disappear within 24 hours. But they can also be chronic.