Sebaceous cysts form when a skin gland or hair follicle becomes blocked, trapping oily secretions beneath the surface. The material builds up over time, creating a slow-growing, firm lump under the skin. Despite the name, most lumps called “sebaceous cysts” are actually epidermoid cysts or pilar cysts, each with slightly different origins but a shared basic mechanism: something prevents skin cells or oil from reaching the surface, so they accumulate in a sac-like pocket instead.
How a Cyst Forms Under the Skin
Your skin constantly produces new cells in its outer layer and sheds old ones. Sebaceous glands, which are tiny oil-producing glands attached to hair follicles, release an oily substance called sebum that keeps your skin and hair lubricated. Normally, dead skin cells and sebum travel up through the follicle and fall away from the surface without you ever noticing.
A cyst forms when that exit route gets disrupted. If the opening to a sebaceous gland becomes blocked, sebum continues to be produced but has nowhere to go. It pools beneath the skin and gradually stretches the surrounding tissue into a rounded sac. In epidermoid cysts, the problem is slightly different: cells from the skin’s outer layer migrate deeper into the tissue instead of shedding outward. Once trapped, these cells keep multiplying and producing a protein called keratin, which fills the cyst with a thick, yellowish, often foul-smelling material.
The cyst wall itself is what makes the lump persist. In an epidermoid cyst, the lining is made of the same cells found on the outer layer of your skin. In a pilar cyst, the lining resembles cells found in hair roots. As long as that lining remains intact, the cyst will continue to refill even if it’s drained.
Blocked Glands and Follicle Damage
The most common trigger is simple obstruction of a sebaceous gland or hair follicle. This can happen for no obvious reason, but several things make it more likely:
- Skin injuries. A scratch, cut, surgical wound, or even chronic sun damage can disrupt the normal path skin cells take to leave your body. This traps cells and keratin beneath the surface, forming a cyst. Cysts can even develop on areas without hair follicles if the skin has been injured.
- Acne. Persistent acne involves exactly this kind of follicle blockage. Cellular debris mixes with sebum, dries out, and creates a plug that obstructs drainage. People with chronic acne are at higher risk for cyst formation, and keeping acne controlled with treatment can reduce that risk.
- Friction or pressure. Repeated irritation to a patch of skin, from tight clothing or habitual rubbing, can damage follicles enough to trigger cyst development over time.
The Role of Hormones
Hormones influence how much sebum your glands produce, which in turn affects how easily they become blocked. During puberty, the body ramps up production of androgens (male hormones present in all sexes), which directly stimulate sebaceous glands to produce more oil. Androgen receptors inside the follicle drive this surge, and the continuous overproduction of sebum can cause the gland to swell and enlarge.
When excess sebum combines with the rapid turnover of skin cells inside the follicle, the mixture can form a plug. This is the same process behind acne, and it explains why cysts often first appear during adolescence or during hormonal shifts like pregnancy or polycystic ovary syndrome. The thicker and more abundant the sebum, the more likely a blockage will develop and persist long enough to become a true cyst.
Genetic Conditions That Cause Multiple Cysts
Most people who develop a cyst get one or two over a lifetime. But if you notice many cysts appearing, especially at a young age or in unusual locations, a genetic condition may be involved. Gardner syndrome is the most well-known example. It’s caused by a mutation on a specific gene (the APC gene on chromosome 5) and produces multiple epidermoid cysts in 50 to 65 percent of affected individuals. These cysts tend to appear around puberty, earlier than typical cysts, and show up in less common spots like the scalp, trunk, and limbs.
Gardner syndrome also involves other growths, including intestinal polyps that carry cancer risk, so the appearance of multiple cysts at a young age is worth mentioning to a doctor. Other inherited conditions can predispose people to recurring cysts as well, though they’re far less common.
Epidermoid vs. Pilar Cysts
The term “sebaceous cyst” gets used as a catch-all, but dermatologists distinguish between two main types based on where in the skin they originate and what their lining looks like.
Epidermoid cysts are the more common variety. They develop when outer skin cells get pushed inward, often through a damaged follicle, and continue to multiply beneath the surface. They appear most often on the face, neck, chest, upper back, and sometimes the scrotum. Pilar cysts originate from cells in the hair root and show up almost exclusively on the scalp. Both types feel similar to the touch: firm, round, and movable under the skin. The distinction matters mainly if you’re having one removed, since the surgical approach and recurrence risk differ slightly.
How to Tell It Apart From a Lipoma
A cyst and a lipoma can both feel like a painless lump under the skin, but they’re fundamentally different. A cyst is a sac filled with trapped skin cells and oil. A lipoma is a slow-growing clump of fat cells. Cysts tend to feel firmer and more defined, sometimes with a small dark spot (a blocked pore) visible at the center. Lipomas feel softer, doughy, and tend to shift easily when you press on them.
In most cases, a doctor can tell the difference just by examining the lump. If there’s any uncertainty, or if the lump is growing quickly or looks unusual, an ultrasound or biopsy can confirm what it is.
When Cysts Become Inflamed or Infected
Sebaceous cysts are typically painless and harmless. The problems start when one gets inflamed or infected. An inflamed cyst becomes red, swollen, warm, and tender. If bacteria enter the cyst, it can fill with pus and become an abscess, which is considerably more painful and may need to be drained.
Squeezing or picking at a cyst is one of the fastest ways to trigger inflammation. Rupture can also happen from accidental pressure or friction, releasing the cyst’s contents into the surrounding tissue and provoking an immune response. If a cyst has recently been inflamed or infected, doctors typically wait at least six weeks for the inflammation to settle before attempting removal. Operating on an actively inflamed cyst increases the risk of wound breakdown and scarring.
Why They Come Back After Drainage
A common frustration with cysts is that they seem to return after being drained. This happens because draining removes the contents but leaves the cyst wall intact. That lining continues to produce keratin and sebum, gradually refilling the sac. Complete removal requires excising the entire cyst wall, which is a minor surgical procedure usually done under local anesthesia. Cysts that have never been infected are easier to remove cleanly, while ones that have been inflamed repeatedly may have scar tissue that makes full excision more difficult.