A boil is a red, painful, lump on the skin that develops at the site of an infected hair follicle. They are also called furuncles.

A hair follicle is a small cavity in the skin from which a hair grows. Boils most commonly develop on areas of skin where there is a combination of hair, sweat and friction, such as the neck, face or thighs.

Over time, pus forms inside the boil. This means it grows larger and becomes more painful. In most cases, a boil will eventually burst and the pus will drain away. This can take from two days to three weeks to happen.

A carbuncle is a collection of boils that develop in a group of hair follicles under the skin. If you have a carbuncle, you may have additional symptoms such as a high temperature and you may feel weak and exhausted.

When to see your GP

Most boils burst and heal by themselves without the need for medical treatment. However, you should visit your GP if you have a boil:

1. on your face, nose or spine 
2. that gets bigger and feels soft and spongy to touch (as it may not burst and heal by itself)
3. that doesn't heal within two weeks

You should also see your GP if you develop a carbuncle or if you have additional symptoms like a high temperature.

You GP should be able to identify a boil or carbuncle by looking at it.

Causes of boils and carbuncles

Boils and carbuncles are often caused by a type of bacteria known as Staphylococcus aureus (staph bacteria). Staph bacteria usually live harmlessly on the surface of the skin or in the lining of the nose. 

However, if they get inside the skin they can trigger skin infections, such as boils. In most cases, staph bacteria enter the skin through cuts and grazes.

Who is affected?

Boils are relatively common in teenagers and young adults, especially in males. Young males living in overcrowded and possibly unhygienic conditions are particularly at risk.

Carbuncles are less common and tend to occur mostly in middle-aged or older men who are in poor health because of a pre-existing health condition, such as diabetes.

Treating boils and carbuncles

Most boils can be successfully treated at home. One of the best ways to speed up healing is to apply a warm facecloth to the boil three or four times a day.

If your boil doesn't heal, your GP may decide to drain it.

Never attempt to squeeze or pierce a boil or carbuncle because it could cause the infection to spread and may lead to complications.

If you develop a carbuncle or there is a high risk of your boil causing complications, you may be prescribed a week-long course of antibiotics.

Some people's boils and carbuncles keep returning. In this case, you may need further tests to discover why this is happening. You may be prescribed an antiseptic solution to remove the bacteria from your skin.

Complications

Although most boils get better without causing further problems, some people develop a secondary infection.

This can range from a relatively minor (though often very painful) infection of the deeper layer of the skin, such as cellulitis, to rarer and more serious infections, such as blood poisoning (sepsis).

Larger boils and carbuncles can also lead to scarring.

Complications are more likely to occur if boils and carbuncles are not treated properly.

Read more about the possible complications of boils and carbuncles.