Urticaria (also known as hives, welts or nettle rash) is a raised, red, itchy rash that appears on the skin. The raised marks in the rash are called weals.

Each weal is short-lived, but the rash can move about. The rash normally disappears after a few days, but can sometimes last longer.

Urticaria happens when a trigger (see below) causes a protein called histamine to be released in the skin. Histamine causes redness, swelling and itching.

When urticaria lasts less than six weeks it is described as acute. If it occurs daily, or more or less daily, for six weeks it is called chronic urticaria. The causes for acute and chronic urticaria are different.

Triggers

Acute urticaria can be triggered by many things, including allergens (such as food or latex), irritants (such as nettles), medicines or physical factors, such as exercise or heat (see Uticaria - causes for a full list).

However, in around half of acute urticaria cases, no cause can be identified.

In most cases of chronic urticaria, the cause is not identified. Unlike acute urticaria, external triggers are not responsible. Chronic urticaria may be linked to other diseases, including autoimmune problems (where the immune system attacks its own tissues).

Who is affected

Acute urticaria is a common condition, and it affects one in five people at some point in their life. It is more likely to affect children.

Chronic urticaria is much rarer, and affects one in every 1,000 people.

Outlook

The symptoms of urticaria are usually short-lived and mild. They can be controlled with antihistamines or, if these do not work, corticosteroid tablets.

Urticaria can be one of the first symptoms of a severe allergic reaction known as anaphylaxis (see Uticaria - complications for more information). Anaphylaxis can cause difficulty breathing, a feeling that you are about to faint, and sometimes loss of consciousness. It is a life-threatening condition and needs emergency treatment.

Most episodes of urticaria peak between 8 to 12 hours, then stop after 24 hours. 

The weals (skin marks) of urticaria are raised, and pink or red in colour. They are usually very itchy and range in size from a few millimetres to the size of a hand.

Individual weals normally fade after a few hours, but can be replaced by new ones elsewhere on the body. They may appear on just one part of the body or across a large part of it. The skin returns to normal as soon as the weal fades.

Urticaria vasculitis

Urticaria vasculitis is a rare form of this condition. The weals are tender, last more than 24 hours and leave a bruise. They need to be urgently looked at by a dermatologist (skin specialist).

Angioedema

Around half of people with chronic (persistent) urticaria and a quarter of people with acute (short-term) urticaria also get a related condition called angioedema.

Angioedema is swelling in the deeper layers of the skin, usually in the eyelids, lips and sometimes the mouth, although it can occur anywhere. Angioedema causes a burning sensation but is not usually itchy. It usually clears within a few days.

For more information, see Health A-Z: angioedema

Acute urticaria

Urticaria results from the release of histamine and other chemicals from under the skin's surface. This causes inflammation (swelling) of the tissues, and fluid to gather under the skin, which causes weals.

The trigger is unknown in around half the cases of acute urticaria. Recognised triggers include:

• an allergen, such as certain foods (e.g. nuts, shellfish or eggs), latex or a wasp sting
• infections, including colds or the flu
• irritants, such as chemicals, nettles or insect bites
• medicines, commonly non-steroidal anti-inflammatory drugs (NSAIDs), such as ibuprofen, which are used to treat pain and fever
• physical triggers, such as pressure to the skin, change in temperature, sunlight, exercise or water

Chronic urticaria

Chronic urticaria is not caused by the same triggers as those for acute urticaria. It may start when your body's immune system attacks its own tissues (called autoimmunity). Antibodies (proteins that usually fight bacteria and viruses) attack the cells in your skin that contain histamine, causing it to be released. This produces urticaria. It is thought that about a third of chronic urticaria cases are autoimmune. It is not known why autoimmune urticaria develops.

In rarer cases, chronic urticaria can also be caused by other chronic illnesses and infections, such as thyroid disease, viral hepatitis or intestinal parasites.

Chronic urticaria often comes and goes. Many people find that certain factors can make it reappear. These include:

• stress
• alcohol
• caffeine
• warm temperature
• prolonged pressure on the skin (for example, through tight clothing)
• medications, especially NSAIDs and codeine

Acute urticaria

Acute urticaria is usually diagnosed by examining the rash on your skin. Your GP will also want to find out what triggered the urticaria, so you can avoid it in future.

They will ask you:

• when and where the episode began
• what you had eaten just before the urticaria started and what you normally eat
• whether you had started any new medication just before the symptoms began
• whether you live or work in an environment where you come into contact with any possible triggers, such as pets, chemicals or latex gloves
• if you had any insect stings or bites just before symptoms started,
• about your current state of health, particularly if you have had any recent infections
• if you have recently travelled to a foreign country and, if so, where
• if there is a history of this condition in your family

A cause is never identified in half of cases.

If your GP strongly suspects a link to an allergic substance, you may be referred to an allergy clinic. Tests may be performed on either your skin or your blood to see if you are allergic to suspected triggers for urticaria, particularly foods and latex.

Chronic urticaria

If your urticaria persists for more than six weeks, it is extremely unlikely that it is due to an allergy, so allergy tests are normally not recommended.

However, your GP should ask about any factors that can make your urticaria worse, such as medicines, your alcohol and caffeine consumption, and stress levels.

You may also be referred for the following tests to see if there is any underlying cause of your chronic urticaria:

• a full blood count test, which can identify anaemia
• a stool sample, which can identify intestinal parasites
• an erythrocyte sedimentation rate (ESR) test, which can help to identify problems with your immune system
• thyroid function tests, which can check whether you have an over- or underactive thyroid gland
• liver function tests, which can check whether you have any problems with your liver.

Acute urticaria

Treatment for acute urticaria may not be required, as the symptoms are mild and the condition can get better on its own within a few days and not return.

If the symptoms of acute urticaria are more serious or the condition persists, you can consult your pharmacist and buy an antihistamine over the counter, or consult your GP for a prescription of antihistamines (see below).

Your GP may also prescribe you corticosteroid tablets if symptoms are severe. Return to your GP if the symptoms get worse or there is no response to treatment after two weeks.

Antihistamines

Antihistamines block the effects of histamine, so they should stop the symptoms of itchiness and reduce the rash. Examples of antihistamines include:

• cetirizine
• fexofenadine
• loratadine

Modern antihistamines do not cause drowsiness in most people, but there are some exceptions. See how you react to the antihistamine before driving or operating heavy machinery. Modern antihistamines may cause drowsiness if taken with alcohol. Always read the information leaflet for your medication.

If you are having problems sleeping at night due to particularly itchy urticaria, your GP may give you additional antihistamines that are known to cause drowsiness, such as chlorphenamine or hydroxyzine.

Antihistamines are normally not prescribed during pregnancy. This is because they have not been established as being entirely safe. However, your GP may recommend chlorphenamine if they feel that the benefits outweigh the risk.

There are several thousand known cases of pregnant women taking chlorphenamine, and there is no evidence that it harms unborn babies.

Corticosteroid tablets

You may be prescribed a short course of high-dose corticosteroid tablets, such as prednisolone. Corticosteroids suppress your immune system and therefore can suppress the symptoms of urticaria.

Possible side effects of corticosteroid tablets include increased appetite and weight gain, mood changes and insomnia.

Chronic urticaria

Treatment for chronic urticaria involves helping you to control your symptoms and avoiding any triggers that make the symptoms worse.

If you have chronic urticaria and angioedema, you should be referred to an immunologist (a specialist in the immune system), an allergist or a dermatologist (a specialist in skin conditions). This is because angioedema is potentially more serious as it can cause breathing difficulties.

If you only have chronic urticaria but the symptoms persist despite treatment, you should also be referred.

Antihistamines

The symptoms of chronic urticaria is treated with antihistamines. You may have to take them regularly for as long as the symptoms last. As with acute urticaria, you may be given a combination of 'non-drowsy' and 'drowsy' antihistamines to help you sleep.

Menthol cream

Menthol cream can be used as an alternative or in addition to antihistamines as it has been shown to relieve itchiness. Your GP can prescribe this.

Corticosteroid tablets

More serious episodes of urticaria can be treated with short doses of corticosteroid tablets, such as prednisolone. Possible side effects of corticosteroid tablets include increased appetite and weight gain, mood change, and insomnia. Long-term use of corticosteroids in chronic urticaria is not recommended.

Diet

There is controversy over the role of diet in people with chronic urticaria. There are two groups of chemicals in foods that may trigger urticaria: vasoactive amines and salicylates. Avoiding or reducing the intake of these chemicals may possibly improve the symptoms.

You may wish to keep a food diary to see if avoiding a certain food helps your symptoms. If you restrict your diet, consult a dietitian, who can make sure that you are not avoiding foods unnecessarily, and that your diet is adequate.

Vasoactive amines:

Foods that contain vasoactive amines, or cause histamine release, include:

• shellfish
• strawberries
• tomatoes
• fish
• chocolate
• pineapple

Salicylates:

Salicylates are naturally occurring aspirin-like compounds that are found in a wide variety of foods of plant origin. You can try cutting down on these but do not completely avoid them. Foods that contain salicylates include:

• tomatoes
• spices
• orange juice
• raspberries
• tea

For more information on following a low-histamine diet, see Allergy UK: histamine intolerance.

Avoiding triggers

If you know what triggers your urticaria or makes it worse, avoiding the trigger may keep your condition under control.

Triggers such as alcohol, caffeine and medication can be easily avoided. Avoiding stress can be harder, particularly if your symptoms negatively affect your quality of life. If you have chronic urticaria, you may find that relaxation techniques such as meditation or hypnosis reduce your stress levels and the severity of your symptoms.

Also monitor the effect of skin creams, soaps and detergents, because they might make your symptoms worse.

Acute urticaria

Around a quarter of people with acute urticaria will also develop acute angioedema (swelling deeper in the tissues) at the same time. Acute angioedema normally gets better within three days. However, dial 999 if you start having difficulty breathing (see box).

Angioedema can be treated with antihistamines and corticosteroid tablets.

Chronic urticaria

Half of chronic urticaria cases go within three to five years. Some cases persist for more than 10 years.

Living with a chronic condition - particularly itchy skin - for a significant amount of time can be frustrating and distressing. If you feel very low for more than a couple of weeks, or you feel you cannot cope or are suicidal, see your GP as soon as possible.

Anaphylaxis

If you have the symptoms of urticaria, watch out for further symptoms that could indicate that you are having an anaphylactic reaction (extreme allergic reaction). If you experience any of the symptoms below, dial 999:

• Swelling of the lips, tongue and the lining of the mouth and throat, which cause difficulty breathing.
• Nausea and vomiting.
• Skin feels cold and clammy.
• A rapid heart rate.
• Feeling faint.
• Sudden feelings of extreme anxiety.

For more information, see Health A-Z: anaphylaxis