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 your doctor if you have any sort of lump so it can be properly diagnosed.
Read more about lumps and swellings.
What a cyst looks like
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 don't 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 don't 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, particularly women. Unlike epidermoid cysts, they run in families.
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.
However, the cells sometimes move deeper into your 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 aren't contagious.
Treating skin cysts
Cysts are usually harmless. Small cysts that aren't causing any problems can be left alone.
Holding a warm flannel against the skin will encourage the cyst to heal and reduce any inflammation.
Don't be tempted to burst the cyst. If it's infected, you risk spreading the infection, and if the sac is left underneath the skin, it can grow back.
See your doctor if you think the cyst is infected – you may need a course of antibiotics.
If a cyst is causing problems, such as catching on your clothes, or if it looks unsightly, it can be removed. Your doctor will use a local anaesthetic to numb your skin, before making a tiny cut and squeezing the cyst out.
This procedure will leave a scar . The cyst may also grow back, particularly if it was removed from the scalp or scrotum.