Blisters (Overview)

What Is It?

A blister is a bubble of fluid under the skin. The clear, watery liquid inside a blister is called serum. It leaks in from neighboring tissues as a reaction to injured skin. If the blister remains unopened, serum can provide natural protection for the skin beneath it. Small blisters are called vesicles. Those larger than half an inch are called bullae. A blood blister is filled with blood, rather than serum.

There are many causes of blisters, including:

  • Irritation — Blisters can be caused by physical factors that irritate the skin, such as friction (rubbing the skin), irritating chemicals or extreme cold or heat. Blisters on the feet can result from shoes that are either too tight or rub the skin in one particular area. Blisters also can be caused by contact dermatitis, a skin reaction to some type of chemical irritant. Intense cold can trigger frostbite, which often leads to blisters once the skin is rewarmed. Any type of burn, even sunburn, also can cause blisters.

  • Allergies — Allergic contact dermatitis, a form of dermatitis or eczema, may result in blisters. Allergic contact dermatitis is caused by an allergy to a chemical or poison, such as poison ivy, poison oak or poison sumac.

  • Infections — Infections that cause blisters include bullous impetigo, an infection of the skin caused by staphylococci (staph) bacteria; viral infections of the lips and genital area due to the herpes simplex virus (types 1 and 2); chickenpox and shingles, which are caused by the varicella zoster virus; and coxsackievirus infections, which are more common in childhood.

  • Skin diseases — Numerous skin diseases cause blisters. Examples include dermatitis herpetiformis, pemphigoid and pemphigus. There also are inherited forms of blistering skin conditions, such as epidermolysis bullosa (in which pressure or trauma commonly leads to blisters) and porphyria cutanea tarda (in which sun exposure provokes blisters).

  • Medications — Many medications, such as nalidixic acid (NegGram) and furosemide (Lasix), can cause mild, blistering skin reactions. Others, such as the doxycycline (Vibramycin), can increase the risk of blistering sunburn by increasing the skin's sensitivity to sunlight. In more dramatic cases, medications can trigger more severe, even life-threatening, blistering disorders, such as erythema multiforme or toxic epidermal necrolysis, also known as TEN, an illness that causes severe skin damage and typically involves 30% or more of the body's surface.

Symptoms

In general, blisters are round or oval bubbles of fluid under the skin that may be painful or itchy, or they may not cause any symptoms. Symptoms vary depending on the cause.

  • Irritation, burns and allergies — Blisters caused by friction or burns are usually painful. Blisters resulting from eczema can be accompanied by redness, severe itching and small bumps on the affected skin.

  • Infections — When blisters are caused by an infection, the symptoms depend on the type of infection. Examples include:

    • Bullous impetigo — The affected skin can redden, and the blisters may burst easily.

    • Herpes simplex virus — When herpes simplex type 1 is the cause, the tiny blisters commonly are known as fever blisters or cold sores. They typically appear on the lips. The affected skin may itch, tingle, swell and become red before the blisters appear. When the blisters eventually break, they leak fluid, and then painful sores develop. Herpes simplex type 2 is the most common cause of genital herpes, a sexually transmitted infection (although type 1 also can cause genital herpes). Generally, small red bumps appear before blisters develop in the affected area, typically the vaginal area or penis, the buttocks and thighs, or the anus. Other symptoms can include fever, muscle aches, headache and burning with urination.

    • Varicella zoster virus — When this virus causes chickenpox, the infection starts with a diffuse, itchy rash that develops quickly into itchy blisters. Varicella zoster also can cause shingles (herpes zoster). People with shingles may experience small, painful blisters that usually erupt in a linear pattern along the length of an infected nerve.

    • Coxsackievirus — Coxsackievirus A16 can cause a condition commonly called hand-foot-and-mouth disease, in which painful blisters often occur on the hands, on the soles of the feet and in the mouth.

  • Skin diseases — Erythema multiforme typically causes blisters on the palms of the hands, the forearms, the soles of the feet, and on the mucous membranes in the eyes, nose, mouth and genitals. Other symptoms include fever, sore throat, cough and muscle pain. The autoimmune diseases (also known as bullous diseases because of the large blisters seen) vary in appearance as well. Dermatitis herpetiformis causes itchy, red bumps or blisters. Pemphigoid, an uncommon condition that primarily affects the elderly, results in large, itchy blisters, and pemphigus, an uncommon disease that tends to strike in middle age, causes blisters inside the mouth and on the surface of the skin. The blisters of pemphigus break easily and leave painful areas.

  • Medications — Reactions to medications vary. In some cases, there is only an increased sensitivity to the sun, which can lead to blistering sunburn if the skin is exposed to the sun. In more severe reactions, such as TEN, blisters can involve larger areas of tissue, including parts of the respiratory passages and gastrointestinal tract, accompanied by fever and malaise (a generally sick feeling).

Diagnosis

If the cause of your blisters is not obvious, your doctor will ask about your family history and your personal medical history, including any allergies you have and any medications you take, including over-the-counter medications. You also will be asked about any recent exposure to irritating chemicals or allergens.

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