Page last reviewed: 13/07/2011

A blister is a small pocket of fluid in the upper layers of the skin. Most blisters are filled with a clear fluid called serum. Serum is the part of the blood that remains after red blood cells and clotting agents have been removed.

A blister usually forms because the outer layer of the skin has been damaged. Fluid collects under the damaged layer of skin, cushioning the tissue underneath. This protects the tissue from further damage and allows it to heal.

Blisters are sometimes filled with blood (called blood blisters) or pus (if they become infected).

A blood blister usually forms when a small blood vessel close to the surface of the skin ruptures (breaks) and blood leaks between the layers of skin. This can happen if the skin is crushed, pinched or tightly squeezed.

Blisters are common and can be caused by:

  • friction to the skin, for example from shoes that rub,
  • contact with chemicals, such as detergent,
  • heat, for example from sunburn or a scald, and
  • medical conditions, such as chickenpox and impetigo (see Useful links).

Most blisters heal naturally and do not require medical attention.


Blood supplies oxygen to the body and removes carbon dioxide. It is pumped around the body by the heart.

Red blood cells

Red blood cells transport oxygen around the body and remove carbon dioxide.


Body tissue is made up of groups of cells that perform a specific job, such as protecting the body against infection, producing movement or storing fat.

Blood vessel

Blood vessels are the tubes in which blood travels to and from parts of the body. The three main types of blood vessel are veins, arteries and capillaries.


A rupture is a break or tear in an organ or tissue.

Page last reviewed: 13/07/2011

Blisters are fluid-filled swellings on the surface of your skin. Some blisters are painless and some feel tender if pressure is applied. Blood blisters are dark in colour and are often more painful than other blisters. You may have a single blister or a group of blisters, depending on the cause.

As new skin grows under a blister, your body slowly reabsorbs the fluid contained in the blister and the outer layer of skin dries and peels off. This normally takes three to seven days.


If your blister is infected, it will fill with pus, which may be yellow or green. The blister may be painful to touch.

The skin around an infected blister may be red or there may be red streaks leading away from the blister. Your skin may feel hot and painful.

Seek medical attention

See your GP if you have blisters that become infected, are very painful, come back frequently or develop in unusual places, such as on your eyelids or the inside of your mouth.

See your GP if you develop blisters after coming into contact with a chemical or other substance.


Blood supplies oxygen to the body and removes carbon dioxide. It is pumped around the body by the heart.
Inflammation is the body's response to infection, irritation or injury. It causes redness, swelling, pain and sometimes a feeling of heat in the affected area.

Page last reviewed: 13/07/2011

Blisters are usually caused by injury to the skin from:

  • heat, for example from sunburn or a scald, or
  • friction, for example from shoes that rub.

Friction or heat on the skin can create a tear between the upper layer of the skin (the epidermis) and the layers beneath. When this happens, the surface of the skin remains intact but is pushed outwards as serum collects in the newly created space between the layers.


If your skin is rubbed for long enough, a blister will form. Short periods of intense rubbing can also cause a blister. Blisters are most common on the hands and feet, which can rub against shoes or handheld equipment, such as tools or sports equipment.

Blisters form more easily on moist skin and are more common in warm conditions.


A blister can form when skin comes into contact with a cosmetic, detergent, solvent or other chemical. 

Blisters can also develop due to an allergic reaction to an insect bite or sting.

Medical conditions

There are a number of medical conditions that cause blisters. The most common are:

  • chickenpox,
  • herpes,
  • impetigo, and
  • pompholyx (a form of eczema).

Other, rarer conditions that cause blisters include:

  • Bullous pemphigoid: a skin disease that causes large blisters. It usually affects people over 60.
  • Pemphigus: a serious skin disease in which blisters develop if pressure is applied to the skin. The blisters burst easily, leaving raw areas that can become infected.
  • Dermatitis herpetiformis: a skin disease that causes intensely itchy blisters, usually on the elbows, knees, back and buttocks. Blisters usually develop in patches of the same shape and size on both sides of the body.
  • Chronic bullous dermatosis of childhood: a disease that causes clusters of blisters on the face, mouth or genitals. 


An allergen is a substance that reacts with the body's immune system and causes an allergic reaction.
Chronic usually refers to a condition that continues for a long time or keeps coming back.

Page last reviewed: 13/07/2011

Blisters from friction or heat will normally have an obvious cause, such as sunburn, shoes that rub or heavy manual work.

Blisters that develop without apparent injury to the skin from heat or friction may be due to a medical condition. If you develop blisters and you are not sure why, see your GP, who will test you for the conditions that may be causing them.

Page last reviewed: 13/07/2011

Most blisters heal naturally and do not require medical attention. As new skin grows beneath the blister, your body will slowly reabsorb the fluid in the blister and the skin on top will dry and peel off.

The unbroken skin over a blister provides a natural barrier to infection. The skin should remain intact to avoid infection. Never pierce a blister with a needle. Allow it to break on its own once the skin underneath has healed.

Cover small blisters with a plaster (adhesive dressing). Larger blisters should be covered with a gauze pad or dressing that you can tape in place. If you have a blister that is painful or in a position that makes it likely to burst (such as on the sole of your foot), cover it with a soft dressing to cushion and protect it. Change the dressing daily.

If a blister bursts, do not peel off the dead skin on top of the blister. Allow the fluid inside to drain, then cover the blister and the area around it with a dry, sterile dressing to protect it from infection until it heals.

Blood blisters

Leave blood blisters to heal naturally. If a blood blister bursts, keep the area clean and dry and protect it with a sterile dressing to prevent infection.

Blood blisters are often painful. An ice pack applied to the area immediately after the injury can help relieve pain. Apply the ice pack for between 10 and 30 minutes. The ice should not touch your skin directly as this may cause a cold burn, so place a towel over the injured area before applying the ice.


If a blister becomes infected, it can be treated with antibiotics, which your GP can prescribe.

Medical conditions

If your blisters were caused by a medical condition, this will need to be treated.


Pain is an unpleasant physical or emotional feeling and your body's way of warning you it has been damaged.
Blood supplies oxygen to the body and removes carbon dioxide. It is pumped around the body by the heart.
Antibiotics are medicines that can be used to treat infections caused by micro-organisms, usually bacteria or fungi. Examples of antibiotics include amoxicillin, streptomycin and erythromycin.

Page last reviewed: 13/07/2011

Blisters that are caused by a medical condition cannot be prevented. However, you can avoid getting blisters from friction, sunburn or chemicals.


Wearing comfortable, well-fitting shoes and clean socks will help you avoid getting blisters on your feet. Blisters are more likely to develop on moist skin. If you have sweaty feet, wearing moisture-absorbing socks or changing your socks twice a day can help prevent blisters. If you are exercising or playing sport, special sports socks can help keep your feet dry and reduce the chance of getting a blister.

If you are going for a long walk, wear shoes that are comfortable and that you have worn before (that have been broken in). If you feel a hot area on your foot when walking, exercising or playing sport, stop immediately and, if possible, tape some padding over the area.

Wearing protective gloves when using tools, such as a shovel or pickaxe, and when doing manual work, such as gardening, will help prevent blisters on your hands.


Wear gloves when handling detergents, cleaning products, solvents and other chemicals.


Use sunscreen and cover your skin with clothing during the hottest part of the day to avoid getting blisters from sunburn. Moisturiser, aftersun or calamine lotion can help to ease the discomfort if you get sunburnt.

Content provided by NHS Choices and adapted for Ireland by the Health A-Z.

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