A cyst is a closed, sac-like pocket of tissue that can form almost anywhere in the body. It typically contains fluid, air, or semi-solid material like keratin (the same protein that makes up your hair and nails). Most cysts are benign, grow slowly, and never cause problems. Some, however, can become painful, infected, or large enough to press on surrounding structures, and those may need medical attention.
What a Cyst Actually Is
Think of a cyst as a small balloon enclosed by its own wall. That wall is made of cells that produce whatever fills the inside, whether that’s clear fluid, thick oily material, or dead skin cells. This self-contained structure is what separates a cyst from an open wound or a simple pocket of inflammation. Because the sac is closed, cysts tend to stay put and grow slowly over time rather than spreading.
Skin cysts, for example, have a layered wall similar to the outer layers of your skin. The innermost cells continuously shed debris into the center of the sac, gradually expanding it. This is why squeezing a skin cyst rarely gets rid of it permanently. Unless the wall itself is removed, it keeps producing material and refills.
Common Types and Where They Form
Cysts show up in two broad categories: those you can see or feel on the surface, and those hidden inside the body.
Cysts on or Near the Skin
- Epidermoid cysts (often called sebaceous cysts) are the most common skin cysts. They appear as firm, round bumps just beneath the skin, usually on the face, neck, or trunk. They occur roughly twice as often in men as in women and typically show up between the ages of 20 and 40.
- Ganglion cysts form along tendons or joints, most often on the wrist. They’re filled with thick, jelly-like fluid.
- Pilonidal cysts develop near the tailbone, frequently from ingrown hairs, and can become painful or infected.
- Baker’s cysts form behind the knee, usually in response to a joint problem like arthritis.
- Chalazia are small cysts on the eyelid caused by a blocked oil gland.
Internal Cysts
- Ovarian cysts are extremely common in women of reproductive age and often form as a normal part of the menstrual cycle.
- Kidney cysts (renal cysts) become more common with age and are usually harmless.
- Breast cysts are fluid-filled sacs that can feel like a smooth, movable lump.
- Pancreatic cysts are sometimes found incidentally during imaging for other conditions.
Why Cysts Form
There’s no single cause. The mechanism depends on the type, but most cysts develop through one of a few pathways.
Blocked ducts or glands. When the opening of a gland gets clogged, secretions build up behind the blockage. The trapped material stretches the surrounding tissue into a sac. This is how many skin cysts, eyelid cysts, and some internal cysts form.
Normal hormonal cycles. Ovarian cysts are often “functional,” meaning they arise from the follicle that releases an egg each month. If the follicle doesn’t rupture during ovulation, or if it seals back up afterward, fluid accumulates and a small cyst forms. These usually resolve on their own within one or two menstrual cycles without any treatment.
Chronic inflammation or injury. Repeated irritation to a tissue can trigger cyst formation. Pilonidal cysts, for instance, are linked to friction and ingrown hairs near the tailbone. In deeper organs, ongoing inflammation can weaken duct walls and cause them to dilate into cyst-like structures over time.
Developmental factors. Some cysts, like dermoid cysts and branchial cleft cysts, form during embryonic development and may not be noticed until years later.
How Cysts Differ From Abscesses and Tumors
A lump under the skin can be unsettling, and it helps to understand what you might be dealing with.
A cyst is a closed sac that grows slowly and is generally painless unless it becomes inflamed. An abscess, by contrast, is a pocket of pus caused by a bacterial infection. Abscesses are typically red, warm, swollen, and tender to the touch. The pus inside is highly contagious, so an abscess needs to be drained and often treated with antibiotics, while a quiet cyst may need no treatment at all.
Tumors are solid masses of tissue rather than fluid-filled sacs. On imaging like ultrasound, cysts appear as dark, hollow circles because fluid doesn’t reflect sound waves the same way solid tissue does. Solid tumors look denser and brighter. Breast cysts, for example, show lower absorption of light compared to surrounding tissue, while solid breast tumors show higher absorption. This difference is one of the main ways doctors distinguish the two without surgery. Most cysts are not cancerous: roughly 1% of epidermoid cysts ever undergo a malignant change.
Symptoms to Pay Attention To
Many cysts produce no symptoms at all and are discovered during a routine exam or imaging scan for something else. When symptoms do appear, they depend on location and size.
A skin cyst usually feels like a smooth, firm marble under the skin. It moves slightly when you press on it and is generally painless unless it becomes infected, at which point the area may turn red, swell, and feel warm. Internal cysts can cause a dull ache or pressure if they grow large enough to push against nearby organs.
Ovarian cysts deserve special mention because they can occasionally rupture or twist the ovary. A ruptured cyst can cause sudden, sharp lower abdominal pain, sometimes with nausea and vomiting. In severe cases, a ruptured cyst can bleed into the abdominal cavity, leading to rapid heart rate, dizziness, or even loss of consciousness from blood loss. Sudden, intense pelvic pain, especially after exercise or sexual intercourse, warrants prompt medical evaluation.
When a Cyst Needs Treatment
A cyst that isn’t causing symptoms generally doesn’t need urgent treatment. Many are monitored with periodic check-ups or imaging, especially internal cysts found incidentally.
Treatment becomes necessary when a cyst is painful, infected, cosmetically bothersome, or shows features that raise concern. For ovarian cysts, doctors typically watch cysts smaller than 10 centimeters (about 4 inches) with follow-up ultrasounds, since most resolve on their own. If a cyst persists through several menstrual cycles, it’s less likely to be a functional cyst and warrants further evaluation. Features like thick internal walls, solid components, or irregular borders on imaging increase the need for closer investigation.
Infected cysts, particularly pilonidal cysts that have formed an abscess, need to be drained surgically. Antibiotics alone won’t clear them. If infections keep recurring, the entire cyst and its wall can be surgically removed to prevent it from coming back.
For skin cysts, the standard approach is a minor outpatient procedure where the cyst is cut out along with its entire wall. Removing just the contents without the wall is why home drainage fails: the sac remains intact and refills. Attempting to pop or cut a cyst at home also risks introducing bacteria, leading to infection or permanent scarring.
Ovarian Cysts and the Menstrual Cycle
Ovarian cysts are common enough that most women will have at least one during their lifetime, often without knowing it. Follicular cysts and corpus luteum cysts are both considered normal parts of the menstrual cycle. A follicular cyst forms when the egg-containing follicle doesn’t release the egg and continues to grow. A corpus luteum cyst forms after the egg is released, when the empty follicle seals off and fills with fluid.
Both types are typically smooth-walled, contain only fluid, and resolve within a few weeks to a couple of months. They rarely cause symptoms unless they grow unusually large or rupture. When a cyst doesn’t go away after several cycles, doctors may recommend blood tests or additional imaging to rule out other conditions, including endometriomas (cysts caused by endometriosis) or, rarely, ovarian cancer. Simple, fluid-filled cysts under 10 centimeters are overwhelmingly benign regardless of age.