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.
Read more about the symptoms of boils and carbuncles.
When to see your doctor
Most boils burst and heal by themselves without the need for medical treatment. However, you should visit your doctor if you have a boil:
- on your face, nose or spine
- that gets bigger and feels soft and spongy to touch (as it may not burst and heal by itself)
- that doesn't heal within two weeks
You should also see your doctor if you develop a carbuncle or if you have additional symptoms like a high temperature.
You doctor 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.
Read more about the causes of boils and carbuncles.
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 doctor 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.
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.
Boils and carbuncles begin as swollen and painful red lumps on the skin, before increasing in size over the following few days.
Boils can develop anywhere on your skin, but they're most likely to occur in places where there's a combination of hair, friction and sweat, such as the:
Over time, boils grow because there is a build-up of yellowish-white coloured pus. The size of boils can vary significantly. Some boils can grow to the size of a golf ball, but most are about the size of a pea.
It's very important to resist the urge to squeeze the boil because it could lead to more serious complications.
Most boils will burst open eventually, allowing the pus to drain and leaving your skin to heal. This can take from two days to three weeks to happen. Most boils don't leave any scarring unless they're particularly large.
A carbuncle is a dome-shaped collection of boils that usually develops over the space of a few days. They most often occur on the back of the neck, back or thighs.
A fully grown carbuncle can range in size from 3cm (1.1 inches) to over 10cm (4 inches), and will leak pus from a number of points. You may also have additional symptoms, such as:
- a high temperature of 38ºC (100.4ºF) or above
- a general feeling of being unwell
- feeling weak and exhausted
When to seek medical advice
Contact your doctor for advice if you have:
- a moderate to large boil that feels soft and spongy to the touch
- a carbuncle
- a boil on your face or spine – this can sometimes cause serious complications
- additional symptoms, such as a high temperature or feeling generally unwell
- a secondary infection, such as cellulitis (an infection of the deeper layer of the skin)
- a boil and a health condition known to weaken the immune system, such as type 2 diabetes or HIV or AIDS
- a boil and you're receiving medical treatment that's known to weaken the immune system, such as chemotherapy
- a boil that shows no sign of healing after two weeks
Most boils and carbuncles develop when the hair follicles in your skin become infected with bacteria.
A hair follicle is a cavity in the skin that a hair grows from.
Boils are usually caused by Staphylococcus aureus bacteria (also known as staph bacteria) infecting one or more hair follicles.
Staph bacteria commonly live harmlessly on the skin and inside the nose and throat. It is estimated that about 20% of otherwise healthy people are long-term carriers of staph bacteria.
Boils tend to occur when the bacteria enters the skin through a cut or graze. This causes your immune system to respond by sending infection-fighting white blood cells to the source of the infection to kill the bacteria.
Over time, a mix of dead bacteria, dead white blood cells and dead skin cells builds up inside the boil to form pus.
Things that make you more likely to get boils include:
- being male (particularly a teenage boy) – this could be because hormonal changes during puberty can make the skin greasy, which may encourage the growth of bacteria
- being in close personal contact with someone who has a boil
- taking part in sports that involve a combination of sweating, close personal contact and frequent friction of the skin, such as rugby and wrestling
- living in conditions that are overcrowded and have poor standards of personal hygiene, or both
- having a pre-existing skin condition, such as atopic eczema or scabies
- obesity – being very overweight with a body mass index (BMI) of 30 or above
However, it is also common for healthy people with good levels of hygiene to develop boils at some point.
Like boils, carbuncles are usually caused by Staphylococcus aureus bacteria. A carbuncle develops when the infection spreads further beneath the skin to create a cluster of boils.
People in poor health or those with a weak immune system (the body's natural defence against infection and illness) are thought most at risk of developing a carbuncle. These include people:
- with diabetes – high sugar levels in the blood make it more difficult for your immune system to protect you against skin infection
- with health conditions known to weaken the immune system, such as HIV
- who are on a long-term course of steroid tablets or injections (corticosteroids) – long-term steroid use makes you more vulnerable to infection
- who are having treatment known to weaken the immune system, such as chemotherapy
- who are malnourished (not getting the nutrients needed from food to maintain good health)
- with skin conditions that affect a large part of their body
- with heart disease
- with a severe drug misuse problem, particularly those who inject drugs
Your doctor will usually be able to diagnose a boil or carbuncle by examining it.
Further testing is usually only required if you have:
- a boil or carbuncle that doesn't respond to treatment – it may be caused by bacteria other than staph bacteria
- multiple boils or carbuncles
- a weakened immune system caused by a condition such as diabetes, or if you're having a treatment such as chemotherapy
In these circumstances, your doctor will probably use a cotton swab to take a tissue sample from the boil so it can be examined under a microscope.
You may also be referred for a blood test to see if you have an undiagnosed underlying health condition, such as diabetes, which could be increasing your risk of developing boils and carbuncles.
Most boils get better without the need for medical treatment and can be successfully treated at home.
One of the best ways to speed up the healing process is to apply a warm facecloth to the boil for 10 minutes three or four times a day. The heat increases the amount of blood circulating around the boil, which means more infection-fighting white blood cells are sent there.
When the boil bursts, cover it with sterile gauze or dressing to prevent the spread of infection. After this, wash your hands thoroughly using hot water and soap. This will help prevent you spreading bacteria to other parts of your body or other people.
See your doctor if you have a larger boil that feels soft and spongy to the touch (the medical name for this is a fluctuant boil).
Once a boil becomes soft and spongy, it is unlikely to burst open by itself and probably won't respond well to treatment with antibiotics.
Your doctor may be able to remove the pus using a technique called incision and drainage. In some cases, your doctor may refer you to your local hospital for this treatment.
Incision and drainage involves piercing the tip of the boil with a sterile needle or scalpel. This encourages the pus to drain out of the boil, which should help to relieve pain and stimulate the recovery process.
Before having the procedure you're likely to be given a local anaesthetic to numb the affected area.
Never attempt to squeeze or pierce a boil yourself because this can spread the infection.
Antibiotics are usually recommended:
- for all cases of carbuncles
- if you have a high temperature
- if you develop a secondary infection, such as cellulitis (an infection of the deeper layer of the skin)
- if you have a boil on your face – facial boils have a higher risk of causing complications
- if you're in severe pain and discomfort
A seven-day course of a penicillin-based antibiotic called flucloxacillin is usually recommended. If penicillin is unsuitable for you, alternative antibiotics, such as erythromycin and clarithromycin, can be used.
It is important to finish the course of antibiotics even if the boil goes away, as not doing so could lead to a return of the infection.
Treating recurrent boils and carbuncles
Boils and carbuncles that keep returning often need further treatment.
Most people with recurrent boils develop them because they are carriers of Staphylococcus aureus bacteria (staph bacteria), a common cause of boils and carbuncles. In this case, treatment may be necessary to kill these bacteria.
Treatment will depend on where the staph bacteria are found on your body. Bacteria on the skin can be treated with antiseptic soap.
Staph bacteria are also commonly found in the nose, in which case you may be prescribed an antiseptic nasal cream to apply several times a day for five to 10 days.
You will also be offered advice about preventing boils, such as regularly washing and cleaning cuts and grazes. See preventing boils and carbuncles for more information.
Although most boils do not cause further problems, this is not always the case.
Scarring can sometimes occur following a larger boil or carbuncle. These scars never disappear completely, but they do fade with time and become less noticeable.
If you're particularly concerned about scars, there are a number of treatment options, including:
- corticosteroid injections, which can help flatten a raised scar
- pressure dressings, which can help flatten and soften a scar
- plastic surgery
However, it's unlikely that your local clinical commissioning group (CCG) will fund these treatments unless it can be shown your scars are causing you considerable psychological distress.
A simpler alternative is to use make-up to conceal any scars you have. Camouflage make-up specially designed for covering scars is available over the counter at pharmacies.
Read more about treating scars.
Spread of infection
The bacteria inside a boil or carbuncle can sometimes spread to other parts of the body and trigger a secondary infection.
Cellulitis is the most common secondary infection associated with boils and carbuncles. It is an infection of the deeper layers of the skin.
Less common secondary infections associated with boils and carbuncles include:
- impetigo – a highly contagious skin infection that causes sores and blisters
- septic arthritis – an infection of a joint
- osteomyelitis – an infection that develops inside a bone
- endocarditis – an infection of the inner layer of the heart
- septicaemia – an infection of the blood
- brain abscess – a collection of pus that develops inside the brain
Some of these less common secondary infections need to be treated with injections of antibiotics. In the case of septicaemia and brain abscess, admission to an intensive care unit (ICU) may be required.
Cavernous sinus thrombosis
Cavernous sinus thrombosis is a very rare but potentially life-threatening complication of a boil. It occurs when an infection triggers a blood clot in the spaces behind the eye socket. The clot begins to increase the pressure on the brain, causing symptoms such as:
- a sharp and severe headache
- swelling of the eyes
- eye pain that's often severe
Without prompt treatment with antibiotics, cavernous sinus thrombosis can be fatal.
Read more about cavernous sinus thrombosis.
It's not always possible to prevent getting a boil or carbuncle, but some simple steps can reduce your risk of developing the condition.
- washing your skin regularly using a mild antibacterial soap
- always carefully cleaning any cuts, wounds or grazes, even if they look very small
- keeping cuts, wounds and grazes covered with a sterile bandage until they heal
- eating a healthy diet and taking regular exercise to boost your immune system – this will make you less likely to develop skin infections, such as boils
Preventing your boil or carbuncle from spreading
If you develop a boil or carbuncle, it's important to prevent spreading the infection to other parts of your body or to other people. You can do this by:
- washing your hands with an antibacterial soap after touching a boil or carbuncle
- washing underwear, bed linen and towels at a high temperature
- using a separate facecloth and towel
- keeping any wounds covered with sterile gauze until they heal
- regularly changing the gauze covering a boil or carbuncle
- sealing used gauze or dressings in a plastic bag and throwing it in the dustbin immediately
- avoiding places like saunas, gyms and swimming baths until your skin has healed