A skin cyst is a fluid-filled lump just underneath the skin. It's common and harmless, and may disappear without treatment.
It can be difficult to tell whether a lump is a cyst or something else that might need treatment.
You should therefore see a GP if you have any sort of lump so it can be properly diagnosed.
Boils and abscesses are painful collections of pus that are caused by bacterial infections. A cyst may go on to become a boil or abscess.
A skin cyst is a round, dome-shaped lump. It's yellow or white, often with a small dark plug through which you might be able to squeeze out pus.
Cysts can range in size from smaller than a pea to a few centimetres across. They grow slowly.
Skin cysts do not usually hurt, but can become tender, sore and red if they become infected.
Foul-smelling pus coming out of the cyst is another sign of infection.
Types of skin cyst
Epidermoid cysts (one of the main types) are commonly found on the face, neck, chest, shoulders or skin around the genitals.
They affect young and middle-aged adults, and are particularly common in people with acne. They do not usually run in families.
Cysts that form around hair follicles are known as pilar cysts. They're often found on the scalp.
Pilar cysts typically affect middle-aged adults, mostly women. Unlike epidermoid cysts, they run in families.
A cyst that forms on the eyelid is called a chalazion or meibomian cyst.
Why do cysts form?
Some of the cells in the top layer of skin produce keratin, a protein that gives skin its strength and flexibility.
Normally, these cells move up to the surface of the skin as they start to die so they can be shed.
But the cells sometimes move deeper into the skin and multiply, forming a sac.
They secrete keratin into the middle of the sac, which forms a thick, yellow paste. This can ooze out of the cyst if it's burst.
Anyone can develop a skin cyst, but you're more likely to have one if you've been through puberty, you have a history of acne, or you've injured the skin (for example, if you've damaged a hair follicle).
Skin cysts are not contagious.
Cysts are usually harmless. Small cysts that are not causing any problems can be left alone.
Holding a warm flannel against the skin will encourage the cyst to heal and reduce any inflammation.
Do not be tempted to burst the cyst. If it's infected, you risk spreading the infection, and it can grow back if the sac is left underneath the skin.
See a GP if you think the cyst is infected. You may need a course of antibiotics.
Although some GP surgeries have minor surgery facilities, most do not remove cysts. You may be referred to a specialist, or you could pay for private treatment.
During a cyst removal, a local anaesthetic is used to numb the skin. A tiny cut is made in the skin and the cyst is squeezed out.
This procedure will leave a scar. The cyst may also grow back, particularly if it was removed from the scalp or scrotum.