An epidermoid cyst is a slow-growing, noncancerous lump that forms just beneath the skin. It develops when skin cells that normally shed from the surface instead get trapped below, creating a small sac that fills with keratin, the same protein that makes up your hair and nails. These cysts are extremely common and almost always harmless, though they can become irritated or infected if left alone long enough or disturbed.
What’s Inside an Epidermoid Cyst
The cyst itself is a small pocket lined with skin cells, sitting within the deeper layer of skin called the dermis. These lining cells continuously produce keratin, which builds up in layers inside the sac. This is why, if a cyst is ever punctured or squeezed, the material that comes out is typically thick, white or yellowish, and has a strong, unpleasant smell. That paste-like substance is compacted keratin, not pus (unless the cyst is infected).
Despite being commonly called “sebaceous cysts,” epidermoid cysts don’t actually contain sebum (oil). True sebaceous cysts, which arise from oil-producing glands, are far less common. The two names get used interchangeably in everyday conversation, but the distinction matters because the contents and the lining of the cyst wall are different. Almost all lumps casually called sebaceous cysts turn out to be epidermoid cysts when examined under a microscope.
Why They Form
Epidermoid cysts develop when surface skin cells migrate inward instead of shedding outward. This can happen for several reasons. Damage to a hair follicle, whether from shaving, waxing, or a minor scrape, can redirect cells beneath the surface. Acne, surgical wounds, and other skin injuries are also common triggers. In some cases, the cyst appears without any obvious cause, simply because a small group of cells ended up in the wrong place during normal skin turnover.
Certain genetic conditions increase the likelihood of developing multiple epidermoid cysts. Gardner syndrome, for instance, is associated with numerous cysts appearing across the body. But for most people, an epidermoid cyst is a one-off or occasional occurrence with no underlying condition behind it.
Where They Typically Appear
Epidermoid cysts can form almost anywhere on the body, but they strongly favor areas with more hair follicles. The face, neck, scalp, and trunk (especially the upper back and chest) are the most common locations. They also frequently show up on the groin and behind the ears. They can range from pea-sized to several centimeters across, and they tend to grow very slowly over months or years.
How to Recognize One
A typical epidermoid cyst feels like a firm, round bump that you can move slightly under the skin with your fingers. The overlying skin usually looks normal. One telltale feature is a small dark dot at the center of the bump, sometimes called a central punctum. This is a tiny, plugged opening that connects to the cyst beneath. Not every cyst has a visible punctum, but when present, it’s a helpful clue.
Most epidermoid cysts cause no pain at all. You might notice one only because you feel it while washing or brushing your hair. They become a problem mainly when they get inflamed or infected, at which point the area turns red, swollen, warm, and tender. An inflamed cyst can grow rapidly over a few days and become quite painful.
Inflammation, Rupture, and Infection
The most common complication is rupture beneath the skin’s surface. When the cyst wall breaks, keratin spills into the surrounding tissue, triggering an intense inflammatory reaction. The area swells, reddens, and hurts, even without bacteria being involved. This sterile inflammation is often mistaken for an infection because it looks so similar.
True bacterial infection can also occur, especially if the cyst has been squeezed or picked at. An infected cyst may produce actual pus, and the surrounding skin can become hot and deeply tender. In rare cases, infection can spread to nearby tissue if left untreated.
Malignant transformation is extremely rare but has been documented in medical literature. Squamous cell carcinoma arising within a longstanding epidermoid cyst is a recognized possibility, though the risk is low enough that routine removal of painless, stable cysts isn’t standard practice for this reason alone. A cyst that suddenly changes in size, shape, or firmness after being stable for years is worth having evaluated.
How They’re Diagnosed
Most epidermoid cysts are diagnosed by appearance alone during a physical exam. The combination of a movable, dome-shaped lump with a central punctum in a typical location is usually enough. If there’s any uncertainty, particularly if the lump is deep, firm, or in an unusual spot, ultrasound imaging can help distinguish it from other types of lumps. When a cyst is surgically removed, it’s often sent for examination under a microscope to confirm the diagnosis. The hallmark finding is a cyst lined with normal-looking skin cells, filled with layers of compacted keratin.
Treatment Options
If a cyst isn’t bothering you, no treatment is necessary. Many people live with small epidermoid cysts for years without any issues. Treatment becomes relevant when a cyst is cosmetically bothersome, located in an area where it gets irritated (like under a bra strap or belt line), or has become inflamed.
For inflamed cysts, a steroid injection directly into the tissue around the cyst can reduce swelling and pain relatively quickly. This calms the inflammation but does not eliminate the cyst itself, so it may flare again later. If infection is present, oral antibiotics may be prescribed. Incision and drainage can relieve pressure from a severely swollen cyst, but again, this is a temporary measure because the cyst wall remains in place.
The only way to permanently get rid of an epidermoid cyst is complete surgical removal of the entire cyst wall. If even a small fragment of the lining is left behind, the cyst will likely regrow. This is why definitive excision is typically done when the cyst is not actively inflamed, since inflammation makes it harder to cleanly separate the cyst wall from surrounding tissue. The procedure is usually minor, performed under local anesthesia in an office setting, and leaves a small scar.
Why You Shouldn’t Pop One at Home
It’s tempting to try squeezing or puncturing a cyst yourself, but this carries real risks. According to Cleveland Clinic, attempting to pop or cut out a cyst at home can introduce bacteria, leading to infection, and often results in worse scarring than a controlled medical procedure would. Even if you manage to express some of the contents, the cyst wall stays intact beneath the skin, and the cyst will refill. Worse, the pressure from squeezing can rupture the cyst wall internally, triggering the painful inflammatory reaction described above. A cyst that was previously painless can become a swollen, angry problem that now requires medical attention.