Sebaceous cysts form when a hair follicle becomes blocked, trapping skin cells and a protein called keratin beneath the surface. The term “sebaceous cyst” is actually a misnomer that has stuck around in everyday language. Most lumps people call sebaceous cysts are technically epidermoid cysts or epidermal inclusion cysts, meaning they’re filled with keratin (a structural protein your skin produces) rather than sebum (the oily substance from oil glands). The causes, though, are well understood.
How a Cyst Forms Under the Skin
Every hair follicle on your body has an opening at the skin’s surface. Normally, dead skin cells travel up through this opening and shed. When that opening gets plugged, the cells have nowhere to go. They accumulate beneath the surface, and the body walls them off with a lining of skin tissue, creating a small sac. That sac slowly fills with keratin, a thick, yellowish, sometimes foul-smelling material. This is the lump you feel under your skin.
The cyst typically maintains a tiny connection to the skin surface through a keratin-filled pore, which sometimes appears as a small dark dot (the punctum). The sac itself is what makes the cyst a cyst rather than just a buildup of debris. As long as that sac remains intact under the skin, it will continue producing and collecting keratin, which is why these lumps grow slowly over months or years.
Specific Triggers That Cause Cysts
Skin Injuries and Trauma
A scratch, cut, surgical wound, or any penetrating injury can push surface skin cells into deeper layers where they don’t belong. Once implanted there, these cells keep behaving normally, producing keratin, but now they’re trapped. The body encapsulates them, forming a cyst. This is why cysts sometimes appear at the site of an old scar or wound, even years later. On parts of the body without hair follicles, skin trauma is the primary way cysts develop.
Acne
People with acne are more likely to develop multiple cysts. Acne involves chronic inflammation and blockage of hair follicles, and cysts can originate directly from comedones (clogged pores). The repeated cycle of follicle disruption gives skin cells more opportunities to become trapped beneath the surface. If you’ve had persistent acne on your face, neck, or back, cysts in those areas are not uncommon.
Chronic Sun Damage
Long-term sun exposure damages the skin’s structure and disrupts the normal pathway skin cells use to shed. This can create the same trapping effect as a physical injury, allowing keratin to accumulate in pockets beneath the surface. Cysts related to sun damage tend to appear on areas with the most exposure, like the face, neck, and forearms.
Genetic Conditions Linked to Cysts
Most people who get a cyst have a single, isolated lump with an identifiable trigger. But developing many cysts, especially at a young age, can sometimes point to an inherited condition. Gardner syndrome is the most notable example. It’s a genetic disorder primarily known for causing polyps in the colon, but it also produces epidermoid cysts, benign bone growths in the jaw and skull, and soft tissue tumors. These skin cysts often appear before the intestinal symptoms do, which makes them a potential early warning sign.
If you have multiple cysts along with other unusual growths, particularly in combination with a family history of colon polyps, that pattern is worth mentioning to a doctor. For the vast majority of people with a single cyst, there’s no genetic component at play.
Where Cysts Typically Appear
Cysts show up most often on the face, neck, upper back, and trunk, all areas dense with hair follicles. They can also form on the scalp (where they’re sometimes called pilar cysts, which have a slightly different lining). Essentially, anywhere you have hair follicles is fair game, and anywhere you’ve had a skin injury is also a possibility. They range from pea-sized to several centimeters across and feel firm and smooth under the skin, like a marble that moves slightly when you press on it.
What Happens When a Cyst Ruptures
Most cysts sit quietly under the skin for years without causing problems. Trouble starts when the sac wall breaks, either from trauma, squeezing, or spontaneously. When keratin leaks into the surrounding tissue, it triggers an intense inflammatory response: redness, swelling, warmth, and pain. This can look and feel exactly like an infection, though it’s often just inflammation from the body reacting to the leaked material.
True infection is also possible, especially if you’ve tried to pop or squeeze the cyst yourself. An infected cyst becomes tender, red, and may drain thick or discolored fluid. Squeezing a cyst at home is one of the most common ways people turn a painless lump into a painful problem.
Why Cysts Come Back After Draining
Simply puncturing a cyst and squeezing out its contents provides temporary relief, but the sac lining remains under the skin. That lining is living tissue that continues to produce keratin, so the cyst refills. This is why simple drainage frequently results in recurrence.
Complete surgical excision, where the entire sac is removed intact, prevents the cyst from returning. A less invasive option called the minimal excision technique uses a small 2 to 3 millimeter incision to express the contents and then extract the sac wall through the tiny opening. This approach requires less cutting and often no stitches, but still removes the structure responsible for regrowth. Whether your doctor recommends full excision or the minimal technique depends on the cyst’s size, location, and whether it’s currently inflamed. Inflamed or infected cysts usually need to calm down before removal, since operating on irritated tissue makes it harder to get the entire sac out cleanly.
Risk Factors at a Glance
- History of acne: Chronic follicle blockage increases the chance of cyst formation, especially on the face, chest, and back.
- Previous skin injuries: Cuts, scrapes, surgical sites, and even repeated friction can implant skin cells beneath the surface.
- Sun damage: Years of UV exposure disrupts normal skin cell shedding.
- Genetics: Conditions like Gardner syndrome cause multiple cysts alongside other growths.
- Age: Cysts are most common in adults, likely because of accumulated skin damage and years of follicle use.
In most cases, there’s nothing you did to cause a cyst, and nothing you could have done to prevent it. A follicle got blocked, skin cells piled up, and the body did what it does: walled the problem off. The lump is almost always benign. If it’s not bothering you, leaving it alone is perfectly reasonable. If it’s growing, painful, or in a visible spot, removal is straightforward and effective when the entire sac is taken out.