Hives (urticaria): causes, red flags and when to seek help

The main symptom, says the NHS, is an itchy rash made up of raised bumps or patches in many shapes and sizes. The rash can appear anywhere on the body, may be in one area or spread widely, and can itch, sting or burn. On white skin it looks pink or red, while on brown and black skin the colour change can be harder to see and the raised, puffy texture may be the more reliable clue.

Individual welts typically come and go, with one patch fading as another appears, which helps distinguish hives from eczema or a drug rash that stays in the same place.

The NHS explains that you get hives when something causes high levels of histamine and other chemicals to be released in the skin, and that this is called a trigger. It says you cannot always prevent hives, but finding out what triggers yours may help avoid an episode.

• Certain foods.

• Contact with certain plants, animals, chemicals or latex.

• Cold, such as cold water or wind.

• Hot, sweaty skin from exercise, emotional stress or eating spicy food.

• A reaction to a medicine, or an insect bite or sting.

• Scratching or pressing on the skin, for example from itchy or tight clothing.

• An infection or a problem with the immune system.

• Water or sunlight, although the NHS says this is rare.

Hives alone are uncomfortable but not dangerous. The risk is that they sometimes accompany a serious allergic reaction. The NHS anaphylaxis information describes a life-threatening allergic reaction needing emergency treatment, and the hives page lists the warning signs below. Angioedema, swelling that develops deeper under the skin, often affects the lips, eyelids and tongue and may accompany hives.

Hives that keep coming back are a different pattern. The NHS suggests you may be allergic to something if they keep returning, and says persistent hives that do not go away with treatment can lead to referral to a skin specialist.

• Call 999 if your lips, mouth, throat or tongue suddenly become swollen.

• Call 999 if you are breathing very fast or struggling to breathe, with wheezing or a feeling of choking or gasping for air.

• Call 999 if your throat feels tight or you are struggling to swallow.

• Call 999 if your skin, tongue or lips turn blue, grey or pale; on brown or black skin this may be easier to see on the palms of the hands or soles of the feet.

• Call 999 if you suddenly become very confused, drowsy or dizzy, someone faints and cannot be woken, or a child is limp, floppy or not responding as they normally do.

• Ask for an urgent GP appointment or get help from NHS 111 if hives do not improve after 2 days, the rash is spreading, you are worried about your child's hives, you also have a high temperature and feel unwell, or you have swelling under the skin, which might be angioedema.

• Seek the same advice if hives keep coming back.

The history focuses on timing and triggers: what you ate, new medicines (including painkillers and antibiotics), recent infections, exercise, heat or cold exposure, contact with latex or animals, insect stings and any swelling or breathing symptoms. A diary of episodes with photographs helps, because the rash often fades before the appointment.

Most acute hives need no tests. If the pattern points to a food, medicine or venom allergy, allergy testing may be arranged through an allergy clinic. For persistent hives, blood tests may be done to look for infection or immune system causes. The NHS says a GP may prescribe menthol cream, antihistamines or steroid tablets, and refer you to a dermatologist if hives do not go away with treatment.

The NHS says a pharmacist can give advice about antihistamine treatment to help a hives rash. Tell the pharmacist about any long-term condition, since you may not be able to take antihistamines, and note that the treatment might not be suitable for young children.

Practical measures include cool compresses, loose cotton clothing, avoiding hot showers and known triggers, and not scratching, which can bring on more welts. Keep a note of what was happening before each outbreak. If an episode follows a new medicine, do not stop prescribed medicine without checking, but report it promptly.

A pharmacist or GP handles most hives. A dermatologist or allergy specialist is involved when hives are persistent, severe, or when a specific allergy needs to be confirmed. Private allergy clinics offer testing without the NHS wait, but results should be interpreted with the history, and any emergency features still need 999.