A blister is a small pocket of fluid that forms in the upper layers of the skin.

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

Most blisters are filled with a clear fluid called serum, which is the part of the blood that remains after red blood cells and clotting agents have been removed. However, blisters are sometimes filled with blood (blood blisters) or pus if they become inflamed or infected. 

Blisters can develop anywhere on the body, but are most common on the hands and feet.

What causes blisters?

Blisters can be caused by:

  • friction to the skin
  • contact with chemicals, such as detergent
  • heat – for example, from sunburn or a scald
  • medical conditions, such as chickenpox and impetigo

Read more about what causes blisters.

When to see your GP

Most blisters heal naturally between three and seven days and don't require medical attention.

See your GP if you have blisters that:

  • you think are infected
  • are very painful
  • keep coming back

An infected blister will be filled with yellow or green pus and may be painful, red and hot.

It's important not to ignore an infected blister because it could potentially lead to secondary impetigo (a contagious bacterial infection of the skin) and further complications, such as cellulitis or sepsis.

You should also talk to your GP if you have blisters in unusual places, such as on your eyelids or inside your mouth, or if they appear after severe sunburn, burns or scalds or an allergic reaction, or after coming into contact with chemicals or other substances.

Read more about treating blisters.

Preventing blisters

There are a number of things you can do to avoid getting blisters caused by friction, sunburn or chemicals. For example, you can:

  • wear comfortable, well-fitting shoes
  • wear gloves when handling chemicals
  • use sunscreen

Read more about preventing blisters.

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