What is a blister?
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.
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).
Diagnosing a blister
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 the sun, irritating chemicals or allergens.
Your doctor often can diagnose the cause of your blisters by their appearance and your history. If your doctor suspects an allergic reaction, he or she may recommend patch tests with chemicals to identify the allergen. Some blistering diseases are diagnosed with a skin biopsy, in which a small piece of tissue is removed and examined in a laboratory.
Expected duration of a blister
How long blisters last depends on their cause. For example, blisters caused by irritation generally go away on their own within a few days, and those triggered by infections and skin diseases can remain for weeks or months. With an autoimmune blistering disorder, blistering may be chronic (long-lasting) and requires ongoing treatment. In skin infections, such as herpes simplex virus infection, the blisters can come back periodically. Blistering skin diseases that result from inherited causes also are long-lasting.
Preventing a blister
There are many simple strategies to prevent blisters caused by skin irritation. You can wear comfortable shoes that fit well, with socks that cushion the feet and absorb sweat. Apply sunscreen to protect your skin from sunburn. Be particularly vigilant about avoiding sun exposure if you are taking medications that are known to cause sun sensitivity, such as doxycycline (sold under many brand names). During cold months, use mittens, hats and heavy socks to protect your skin against freezing temperatures and chilling winds.
As much as possible, avoid irritants and allergens that tend to trigger eczema, such as certain hygiene products (bubble baths, feminine hygiene sprays, detergents), certain metals in jewelry, especially nickel, and irritating plants such as poison ivy.
To prevent blisters caused by infections, wash your hands often and never touch skin sores, cuts or any open or broken areas of skin on other people. To reduce the risk of herpes simplex, never have sex (even with a condom) with someone with active herpes. In addition, the fewer sexual partners you have, the lower your risk of herpes simplex. To avoid the spread of childhood infections, try to prevent children from sharing toys and utensils that have touched another child's mouth.
To prevent chickenpox and to help prevent shingles later, have your child immunized with the varicella vaccine. People who have not had chickenpox should avoid people with chickenpox or shingles until all of the blisters have crusted over. Adults can be vaccinated if they have never had chickenpox, especially if they are at risk of exposure (for example, day-care workers and teachers). Shingles vaccination is routinely recommended for adults age 50 and older.
There is no known way to prevent many of the blistering diseases such as the hereditary forms and the autoimmune (bullous) diseases.
Treating a blister
Usually, it is best to leave blisters alone. Because blisters protect the underlying skin, breaking blisters open can increase the chance of infection. Protect blisters with a bandage and cover them until they heal on their own. The liquid in the blister will be re-absorbed and the skin will flatten naturally. If a blister breaks, wash the area with soap and water, then apply a bandage. If a blister is very large or painful, your doctor may drain it and apply an antibacterial cream to prevent infection.
The treatment for blisters caused by eczema, infections and other diseases varies. Some cases of eczema can be treated with corticosteroid cream or pills. Herpes simplex infections and shingles (herpes zoster) sometimes are treated with antiviral medications. Antibiotic cream or pills may be given for impetigo. Chickenpox and coxsackievirus generally are left to go away on their own. The itching caused by chickenpox can be relieved with over-the-counter anti-itch lotions, such as calamine. With medication-related erythema multiforme, the medication must be discontinued immediately. Corticosteroids sometimes may be prescribed.
Pemphigoid and pemphigus are treated with corticosteroids and/or other immunosuppressive agents. Because dermatitis herpetiformis is associated with celiac sprue (a condition that develops as an immune reaction to gluten in the diet), people with dermatitis herpetiformis may benefit from a diet that does not contain any gluten (a substance found in most grains). Porphyria cutanea tarda can be treated with regular removal of blood (phlebotomy) or with medications, including hydroxychloroquine or chloroquine. Some inherited skin disorders that cause blistering may respond to measures that protect the skin from trauma.
When to call a professional
Call your doctor whenever you have blisters of unknown cause, very painful blisters, or blisters accompanied by other symptoms such as fever and malaise (a generally sick feeling). Also call your doctor if a blister develops signs of infection, such as increasing redness, red streaks in nearby skin, oozing blood or pus, increased pain or swelling of the surrounding skin.
In many cases, blisters will disappear when the cause is removed or the infection has gone away, usually in a matter of days or weeks. In most herpes infections, blisters can return in the same spot (such as fever blisters on the lips or genital herpes) weeks, months or even years after the first blisters appear. Pemphigoid and pemphigus are typically chronic (long-lasting) and require long-term therapy.
National Institute of Arthritis and Musculoskeletal and Skin Diseases
Centers for Disease Control and Prevention (CDC)