Cysts under the skin form when skin cells that normally shed from the surface instead move deeper into tissue, where they create a small sac and fill it with a protein called keratin. This trapped material slowly accumulates, producing a firm, round lump that can range from pea-sized to several centimeters across. The process is usually painless and slow, sometimes taking months or years before the cyst becomes noticeable.
How Skin Cysts Actually Form
Your skin constantly produces and sheds cells from its outermost layer. Normally, these cells flake off the surface without issue. But sometimes they migrate inward instead of outward, embedding themselves in the deeper layers of skin. Once there, these displaced cells form the walls of a tiny sac and begin secreting keratin, the same tough protein that makes up your hair and nails. The keratin has nowhere to go, so it builds up inside the sac, and the lump gradually grows.
Several things can set this process in motion:
- Skin trauma or injury: A cut, scrape, surgical wound, or even chronic friction can push surface cells beneath the skin, where they become trapped.
- Hair follicle damage: Inflammation, irritation, or blockage of a hair follicle is one of the most common triggers. When the opening of a follicle gets clogged or disrupted, the cells lining it can start forming a cyst wall instead of functioning normally.
- Developmental factors: A small percentage of cysts form before birth, when clusters of skin cells get trapped during embryonic development. These may not become apparent until later in life.
- Viral infection: Certain strains of HPV have been linked to cyst formation in some cases.
Types of Cysts That Grow Under the Skin
The term “sebaceous cyst” is technically a misnomer. Most lumps people call sebaceous cysts are actually one of two types, and neither involves the oil-producing (sebaceous) glands the name implies.
Epidermoid cysts are by far the most common. They originate from the upper portion of the hair follicle and typically appear as flesh-colored to yellowish, firm, round bumps. Many have a small dark dot at the center, called a punctum. If squeezed or drained, their contents are soft, cheese-like, and notably foul-smelling. These can show up almost anywhere on the body but are especially common on the face, neck, and trunk.
Pilar cysts (also called trichilemmal cysts) originate from the hair root sheath and occur most often on the scalp. They look and feel similar to epidermoid cysts but tend to be smoother, lack that central punctum, and contain firmer, white material. They’re also easier for a surgeon to remove in one piece.
Who Gets Skin Cysts
Skin cysts can develop at any age, but they most commonly appear in adults. Anyone with skin and hair follicles is susceptible, which is essentially everyone, though people who experience frequent skin irritation, shaving-related inflammation, or acne may be at slightly higher risk simply because their follicles endure more damage.
Having multiple cysts, especially appearing around puberty, can occasionally signal an inherited condition called Gardner syndrome. In this genetic disorder, 50 to 65 percent of affected people develop epidermoid cysts, often in less typical locations like the scalp, trunk, and limbs. Gardner syndrome also causes intestinal polyps and other growths, so multiple cysts appearing at a young age are worth mentioning to a doctor even if the cysts themselves seem harmless.
What an Infected or Ruptured Cyst Feels Like
Most cysts sit quietly under the skin for years without causing trouble. Problems start when a cyst becomes infected or ruptures internally. An infected cyst swells rapidly, becomes tender or warm to the touch, and the surrounding skin turns pink, red, or darker than your natural skin tone. If the cyst ruptures, either from pressure or on its own, it can leak thick, yellow, foul-smelling fluid and trigger significant pain and inflammation as the keratin contents spill into surrounding tissue.
The body treats ruptured cyst material as a foreign invader, mounting an inflammatory response that can make the area look and feel much worse than the original lump. This is one reason squeezing or popping a cyst at home is a bad idea: you’re more likely to rupture it inward than to clear it out, and you risk pushing bacteria deeper into the skin or causing severe bleeding.
Cysts vs. Other Lumps Under the Skin
Not every bump beneath the skin is a cyst. Lipomas, the other extremely common subcutaneous lump, feel distinctly different. A lipoma is a collection of fat cells that forms a soft, doughy mass you can easily move around with your fingers. They’re almost always painless and typically stay under two inches in diameter. A cyst, by contrast, feels firmer, is more anchored in place, and may have that telltale central punctum.
An abscess can look similar to an inflamed cyst but tends to develop faster, feels hot, and is usually caused by a bacterial infection from the start rather than by trapped keratin. Abscesses are more likely to need prompt drainage and antibiotics.
Any lump that is larger than two inches (roughly the size of a golf ball), feels hard or stiff, grows rapidly, bleeds, or develops open sores warrants professional evaluation. These characteristics don’t automatically mean something serious, but they fall outside the typical profile of a benign cyst.
How Skin Cysts Are Treated
A cyst that isn’t bothering you often doesn’t need treatment at all. For cysts that are inflamed or uncomfortable, applying a warm, wet washcloth for 20 to 30 minutes, three to four times a day, can help reduce swelling and encourage drainage. Keep the water at a comfortable bath temperature to avoid burning the skin.
When a cyst needs medical attention, there are two main approaches. Simple drainage involves a provider making a small opening and expressing the contents, which provides quick relief but is not a permanent fix. Because the sac lining stays in place, the cyst frequently refills over weeks or months. Surgical excision removes the entire cyst, including its wall, and significantly lowers the chance of recurrence. If the sac is left behind, even partially, the cyst can come back.
For infected cysts, a doctor may drain the infection first and prescribe a course of treatment before scheduling excision, since operating on actively inflamed tissue increases the risk of incomplete removal. The excision itself is typically a brief outpatient procedure done under local anesthesia, with a small scar as the main tradeoff.
Why Cysts Come Back
Recurrence is the single most frustrating aspect of skin cysts. If a cyst is drained but the lining remains intact beneath the skin, it functions like a balloon that’s been deflated: the structure is still there, ready to fill up again. Complete surgical removal of the sac is the only reliable way to prevent this. Even then, new cysts can form in different locations, especially if you’re prone to follicle blockages or have an underlying genetic predisposition. A cyst that keeps returning in the same spot after drainage is a strong candidate for full excision.