Blisters: Causes and Treatment

What blisters are and why they form

The skin has an outer layer (epidermis) and a deeper layer (dermis). When the outer layer is damaged by rubbing, heat or inflammation, the layers separate and fluid from the surrounding tissue fills the gap. The fluid cushions the damaged skin underneath and protects it from infection while new skin grows.

Blister fluid is usually clear. A blood blister forms when small blood vessels are crushed, for example from pinching a finger. Cloudy, yellow or green fluid suggests infection.

Common causes of blisters

Friction

Repeated rubbing from shoes, sports, tools or rowing creates blisters on the heels, toes, palms and fingers. Heat and moisture make them more likely, which is why they are common in hiking, running and new shoes.

Burns, sunburn and cold

• Burns: blisters mean a partial-thickness (second-degree) burn.
• Sunburn: blistering signals a more severe burn; avoid further sun and watch for fever and chills.
• Frostbite: blisters appear as frozen skin rewarms.

Contact dermatitis and bites

Contact dermatitis from poison ivy, oak or sumac causes itchy blisters in streaks or lines where the plant touched. Chemicals, metals and some creams can do the same. Insect bites, particularly fleas, bed bugs and some ant stings, can form blisters, especially on the legs.

Eczema

Dyshidrotic eczema causes intensely itchy, deep-seated "tapioca" blisters on the sides of the fingers, palms and soles. Flares are linked to stress, sweating, hot weather and nickel.

Blisters from infections

Vulvar vesicles

Small blisters on the vulva (vulvar vesicles) are most often caused by genital herpes. They usually start with tingling or burning, then form painful clusters that open into shallow sores within days. Other causes include contact dermatitis from soaps or products, yeast infections with tiny satellite spots, and friction. A swab taken from a fresh blister confirms herpes; see a doctor or sexual health clinic while blisters are present.

Less common but serious causes

• Bullous pemphigoid: large, tense, itchy blisters, mainly in people over 70. An autoimmune condition.
• Pemphigus: fragile blisters and raw areas, often starting in the mouth.
• Dermatitis herpetiformis: intensely itchy small blisters on the elbows, knees and buttocks, linked to celiac disease.
• Stevens-Johnson syndrome and toxic epidermal necrolysis: severe drug reactions with blistering, skin peeling and mucous membrane sores.
• Epidermolysis bullosa: rare inherited conditions where skin blisters with minimal friction from birth.
• Porphyria cutanea tarda: blisters on sun-exposed backs of hands, with fragile skin.

How doctors find the cause

Most friction and burn blisters need no tests. For unexplained blistering, a doctor will ask where and when the blisters appeared, whether they itch or hurt, and about medicines, contacts, illness and family members. A swab of blister fluid tests for herpes or chickenpox virus. A skin biopsy with special staining (immunofluorescence) diagnoses autoimmune blistering diseases. Blood tests may check for celiac disease or porphyria.

How to treat a blister

Should you pop a blister?

Usually no. The roof of the blister is the best protection against infection. Leave it intact and protect it:

1. Cover it with a soft dressing or a hydrocolloid blister plaster, which cushions it and promotes healing.
2. For pressure areas, use a ring-shaped (donut) pad around the blister.
3. Change dressings if they get wet or dirty.

Draining a large, painful friction blister

If a friction blister is large, tense and painful in a spot that will burst anyway, some guidelines allow careful draining:

1. Wash your hands and the blister with soap and water.
2. Sterilize a needle with rubbing alcohol.
3. Puncture the edge of the blister in one or two spots and let the fluid drain.
4. Leave the roof in place. Apply petroleum jelly and a non-stick dressing.

Do not drain blisters from burns larger than a coin, herpes, chickenpox or unexplained causes, and do not drain blisters if you have diabetes or poor circulation.

If it bursts

Wash gently with soap and water, smooth the skin flap down without peeling it off, apply petroleum jelly, and cover it. Remove the dead skin only once the skin underneath has healed.

Treating the cause

• Burns: cool under running water for 20 minutes, then cover.
• Dermatitis: avoid the trigger; a steroid cream may be prescribed.
• Herpes and shingles: antiviral tablets, best started early.
• Impetigo: antibiotic cream or tablets.
• Autoimmune blistering: specialist treatment, often with steroids or other immune-modifying medicines.

Preventing friction blisters

• Wear well-fitting shoes and break new ones in gradually.
• Use moisture-wicking socks, or two thin pairs, rather than cotton.
• Apply tape, moleskin or a lubricant to hot spots before activity.
• Wear gloves for gardening, rowing or tool work.
• Stop and protect any area that feels warm or sore before a blister forms.

Frequently asked questions

What causes small blisters on the skin?

Small itchy blisters are often from contact dermatitis, dyshidrotic eczema or insect bites. Clusters of painful small blisters suggest herpes or shingles, and blisters in crops all over the body with fever suggest chickenpox. Unexplained or spreading blisters should be checked by a doctor.

Should you pop a blister?

Generally no. An intact blister protects the skin underneath from infection. If a friction blister is large and painful, it can be drained carefully with a sterilized needle at the edge, leaving the roof in place. Do not drain burn, herpes or unexplained blisters.

What are vulvar vesicles?

Vulvar vesicles are small fluid-filled blisters on the vulva. The most common cause is genital herpes, which typically causes painful clusters that burst into sores. Contact dermatitis, yeast infections and friction are other causes. A swab from a fresh blister can confirm the diagnosis.

How long do blisters take to heal?

Friction blisters usually heal in 3 to 7 days if protected. Burn blisters take 1 to 3 weeks depending on depth. Blisters from infections heal as the infection settles, often in 1 to 2 weeks. A blister that is not healing or looks infected needs a doctor's review.

When is a blister serious?

See a doctor for blisters with signs of infection, widespread blistering, blisters with fever or mouth sores, large burn blisters, blisters on the feet in diabetes, and blisters that keep coming back without a clear cause.