Sebaceous cysts form when skin cells that normally shed from your body’s surface get trapped beneath the skin instead, creating a small pocket that fills with a protein called keratin. Despite the name, most of these cysts have nothing to do with your oil glands. The term “sebaceous cyst” is actually a misnomer: the correct name is epidermoid cyst, because the cyst wall is made of skin cells, not oil-producing tissue. The thick, yellowish material inside is compacted keratin and cell debris, not sebum (the oily substance your skin produces for moisture).
What Actually Happens Under the Skin
Your skin constantly produces new cells in its deeper layers, pushing older cells to the surface where they flatten, die, and flake off. Sometimes this orderly process goes wrong. Cells that should migrate upward instead move deeper into the skin, settling into the second layer (the dermis). Once trapped there, these cells do what skin cells do: they form a small enclosure and keep producing keratin. That keratin has nowhere to go, so it accumulates inside this self-made sac, and the lump slowly grows.
The cyst wall itself is made of the same type of cells that line the outermost layer of your skin. Think of it as a tiny bubble of surface skin buried in the wrong place, continuously filling with the same protein your skin normally sheds.
Common Triggers
The most frequent starting point is a blocked hair follicle. A blockage at the top of the follicle traps skin cells underneath, and that’s where the cyst begins. This is why these cysts show up most often in areas with dense hair follicles: the face, neck, behind the ears, and the trunk.
Skin injuries are another major trigger. A scratch, surgical wound, or even chronic acne can disrupt the normal path skin cells take to reach the surface. The disruption traps cells and keratin below, where they accumulate into a cyst. On parts of the body without hair follicles, like the palms and soles, trauma can push surface skin cells into the dermis directly, creating a pocket where keratin collects. Cysts on the hands and feet have also been linked to certain strains of HPV.
Long-term sun damage plays a role too, particularly in older adults. Years of UV exposure thins the outer skin layer and makes it more fragile, which makes it easier for cells to get displaced into deeper tissue. This accumulated damage to hair follicles and surrounding structures can set the stage for cyst formation.
Where They Typically Appear
Epidermoid cysts show up most often on the face, neck, the area around the ears, and the torso. They strongly favor hair-bearing skin, though they can form almost anywhere, including mucous membranes, under fingernails, and rarely even inside bones or the brain. Cysts that appear in unusual locations, like along the midline of the body or in genital areas, sometimes trace back to how skin layers fused together during embryonic development, trapping small clusters of skin cells in the process.
Genetic Conditions That Increase Risk
Most people who develop a cyst get one or two over a lifetime and have no underlying condition. But when someone develops many cysts, especially at a young age or in unusual locations, a genetic syndrome may be involved. Gardner syndrome, a severe form of a hereditary intestinal condition, is one well-known example. People with this syndrome develop multiple skin cysts alongside other growths. In these hereditary cases, cysts tend to appear earlier in life and in less typical spots on the body.
What Happens When a Cyst Ruptures
Left alone, most epidermoid cysts are painless and harmless. Problems start when one ruptures, either from pressure, squeezing, or just friction from clothing. When the cyst wall breaks open beneath the skin, all that accumulated keratin and cell debris spills into the surrounding tissue. Your immune system treats this material as a foreign invader, triggering an intense inflammatory response. The area becomes red, swollen, warm, and painful, sometimes rapidly. This inflammation can look and feel like an infection, though bacteria aren’t always involved.
A ruptured cyst can also become genuinely infected if bacteria enter through the compromised skin. At that point, you may notice pus, increasing redness that spreads outward, or fever. This is why squeezing or trying to pop a cyst at home tends to make things worse rather than better.
How They’re Removed
Simply draining a cyst provides temporary relief but almost guarantees it will return, because the cyst wall remains intact beneath the skin and continues producing keratin. Permanent removal requires taking out the entire sac. Two common surgical approaches, a small punch incision and a traditional elliptical cut, both achieve recurrence rates around 3%. The key in either method is removing the cyst wall completely. If even a small fragment remains, the cyst can regrow.
Removal is typically a quick office procedure under local numbing. Recovery involves keeping the area clean while the small wound heals, which usually takes a couple of weeks. Cysts that are actively inflamed or infected are harder to remove cleanly, so your provider may treat the inflammation first and schedule the excision once things calm down.
Why the Name “Sebaceous Cyst” Persists
True sebaceous cysts, ones actually filled with the oily sebum your skin produces, do exist but are rare. They’re associated with a specific skin condition called steatocystoma multiplex. The vast majority of lumps people call sebaceous cysts are epidermoid cysts filled with keratin, not oil. The misnomer has stuck around because it’s been used casually for decades, and many providers still use it out of habit. If you’ve been told you have a sebaceous cyst, there’s a very high chance it’s actually an epidermoid cyst. The distinction matters little for treatment, but it explains why the lump’s contents look like thick, cheesy material rather than clear oil.