Epidermoid cysts form when skin cells that normally shed from the surface get trapped beneath the skin instead, creating a small pocket that fills with a soft, cheese-like protein called keratin. This can happen spontaneously, after an injury, or because of a genetic condition. They’re the most common type of skin cyst, and despite often being called “sebaceous cysts,” they have nothing to do with oil glands.
How Epidermoid Cysts Form
Your skin constantly produces new cells in its outer layer and sheds old ones from the surface. An epidermoid cyst develops when some of those surface-layer cells migrate inward, beneath the skin, instead of sloughing off. Once trapped, these cells continue doing what they’ve always done: producing keratin, the tough protein that makes up your outer skin, hair, and nails. But with nowhere to go, the keratin accumulates inside a sac, slowly expanding into a round, firm lump just under the skin.
The cyst wall is essentially a pocket of normal skin turned inside out. It keeps producing keratin the way your outer skin does, which is why these cysts tend to grow slowly over months or years. The material inside is thick, yellowish-white, and often has a strong odor if the cyst ruptures or is squeezed.
Blocked Hair Follicles
The most common trigger is a disruption to a hair follicle. Each hair follicle is a tiny tube lined with the same type of cells that cover your skin’s surface. If the follicle opening gets blocked or damaged, whether from inflammation, friction, or acne, those lining cells can get pushed deeper into the surrounding tissue. Once there, they form the walled-off pocket that becomes a cyst. This is why epidermoid cysts appear most often on the face, neck, upper back, and chest, areas dense with hair follicles and prone to acne or irritation.
Skin Injuries and Surgery
Penetrating injuries can physically implant surface skin cells into deeper tissue. A splinter, a cut, a needle stick, or a surgical incision can all push fragments of the skin’s outer layer beneath the surface, where they seed cyst formation. These trauma-related cysts can show up in locations where epidermoid cysts don’t usually appear on their own, such as the palms, soles of the feet, and buttocks. The cyst may not become noticeable until weeks or months after the injury, since it takes time for the trapped cells to produce enough keratin to form a visible lump.
Genetic Conditions
Most people who get an epidermoid cyst have one or two at most, and there’s no underlying genetic issue. But developing many cysts, especially at a young age, can signal a hereditary syndrome.
Gardner syndrome is the most well-known genetic link. It’s caused by a mutation that leads to growths throughout the body, including multiple epidermoid cysts, polyps in the colon, and bony tumors. The cysts sometimes appear before the intestinal polyps do, making them a potential early warning sign.
Gorlin syndrome, a rare inherited condition driven by mutations in a gene called PTCH1, also carries a strong association. Epidermoid cysts have been reported in 50% or more of patients with Gorlin syndrome. This condition primarily causes multiple basal cell skin cancers starting at a young age, along with skeletal abnormalities and other systemic features.
Why They’re Not “Sebaceous Cysts”
Epidermoid cysts are routinely called sebaceous cysts, but this is a misnomer that persists in everyday language. True sebaceous cysts arise from oil-producing glands and are filled with an oily substance called sebum. They’re actually quite rare. Epidermoid cysts, by contrast, are lined with skin cells and filled with keratin. The distinction matters because the two have different structures and, in rare cases, different implications. If you’ve been told you have a sebaceous cyst, it’s almost certainly an epidermoid cyst.
What Makes Them Inflamed or Infected
An epidermoid cyst can sit quietly under your skin for years without causing problems. Trouble starts when the cyst wall ruptures, usually from squeezing, friction from clothing, or minor trauma. When keratin leaks into the surrounding tissue, your immune system reacts to it as a foreign substance, triggering redness, swelling, warmth, and pain. This inflammatory response is often mistaken for a bacterial infection, though true infection can also occur, particularly after attempted home drainage with unsterile tools.
Inflamed cysts can double in size within days and become quite tender. The inflammation typically settles on its own or with warm compresses, but an infected cyst may need to be drained and treated with antibiotics.
Risk of Cancer
Epidermoid cysts are benign, and the chance of one turning cancerous is extremely low. Published rates of transformation into squamous cell carcinoma range from 0.011% to 0.045%. This is rare enough that routine removal of painless, stable cysts purely for cancer prevention isn’t recommended. That said, a cyst that suddenly grows rapidly, becomes fixed to deeper tissue, or looks unusual on imaging warrants evaluation.
Treatment and Recurrence
Draining a cyst provides temporary relief but doesn’t solve the problem. The cyst wall remains intact under the skin and will refill over time. Permanent removal requires excising the entire cyst wall. One common approach uses a small punch tool to create a 3 to 4 millimeter opening, through which the cyst contents are squeezed out and the deflated sac is pulled through and cut away. This technique carries a recurrence rate of about 6%. Traditional excision with a wider incision has an even lower recurrence rate but leaves a slightly larger scar.
Cysts that aren’t bothering you don’t need treatment. Many people live with small, stable epidermoid cysts indefinitely. Removal is typically pursued when a cyst becomes repeatedly inflamed, is in a cosmetically sensitive area, or is large enough to be uncomfortable.