The inside of a sebaceous cyst is mostly keratin, a tough protein your skin naturally produces. Mixed in with the keratin are lipids (fats) and other skin cell debris, all packed together into a thick, paste-like material that ranges from white to yellowish in color. The substance often has a strong, unpleasant smell, sometimes compared to rancid cheese.
What the Material Actually Is
Your skin constantly sheds dead cells and produces oils. Normally these materials reach the surface and wash away. Inside a cyst, they have nowhere to go. Keratin, the same structural protein that makes up your hair and nails, accumulates and compresses into a dense, semi-solid mass. The lipids mixed in give the contents a greasy, sometimes cottage cheese-like texture. Over months or years, the cyst slowly fills with more of this material, which is why these lumps tend to grow gradually rather than appearing overnight.
If the cyst ruptures or is squeezed, what comes out is typically a thick, yellowish or white paste with that characteristic foul odor. The smell comes from the breakdown of keratin and fatty acids trapped in an enclosed space for extended periods.
The Sac That Holds It All Together
The contents don’t just float freely under the skin. They’re enclosed in a thin, balloon-like sac made of skin cells. This lining is what continuously produces the keratin filling. Think of it as a tiny, self-contained pouch of skin that turned inward instead of facing outward. The sac wall is the reason these cysts keep coming back if they’re only drained. As long as the lining remains, it will keep generating new material and refill.
Many of these cysts have a small dark dot on the surface called a punctum. This is a tiny opening that connects the sac to the skin’s surface, essentially the remnant of the hair follicle opening where the cyst originally formed.
Why “Sebaceous Cyst” Is a Misleading Name
Despite the name, most so-called sebaceous cysts don’t actually involve the oil-producing (sebaceous) glands at all. The more accurate term is epidermoid cyst. These cysts develop from the upper part of a hair follicle, not from the gland that makes sebum. The contents are primarily keratin from skin cells rather than the oily sebum you might expect from the name. True sebaceous cysts, called steatocystomas, do exist but are far less common. When most people and even many doctors say “sebaceous cyst,” they’re really talking about an epidermoid cyst.
This distinction matters if you’re trying to understand what’s inside. The filling is closer to compacted dead skin than to the oil your pores normally produce.
What Happens When the Contents Escape
An intact, uninfected cyst is generally painless. Problems start when the sac wall breaks, either from pressure, trauma, or attempted squeezing. When keratin leaks into the surrounding tissue, your immune system treats it as a foreign invader. This triggers an intense inflammatory reaction: redness, swelling, warmth, and tenderness around the lump. The cyst can double in size within days.
Inflammation and infection are two different things, though they can look similar. An inflamed cyst is your body reacting to its own leaked keratin. An infected cyst means bacteria have colonized the contents, producing additional swelling, pain, skin discoloration, and drainage of yellow, foul-smelling fluid. Not every cyst becomes infected, but squeezing or popping one at home significantly raises the risk by introducing bacteria through broken skin.
Why Draining Alone Doesn’t Work
Popping or draining a cyst removes the contents but leaves the sac behind. Since the lining is what produces the keratin in the first place, the cyst simply refills over weeks to months. Permanent removal requires taking out the entire sac wall. This is typically done through a small surgical excision.
Some approaches involve first draining the cyst contents, which causes the sac to collapse and shrink. The smaller, deflated sac is then easier to dissect out completely through a smaller incision a few weeks later. The key principle is the same either way: if any fragment of the lining stays behind, recurrence is likely. Complete removal of the sac is what prevents the cyst from coming back.