Cysts are abnormal sacs containing fluid, air, or semisolid material that can develop in skin or tissues in any part of the body. While they may form on internal organs and there are hundreds of types of cysts, this information will focus on those that are located on the skin (epidermoid cysts). These types of cysts normally grow slowly, are painless, and are found on the face, neck, or trunk of the body.

The epidermis is the thin, outer protective layer of skin that constantly sheds cells. Most epidermoid cysts occur when these cells move deeper into the skin instead of sloughing off like normal. They may also form after injury or irritation to the skin or portion of a hair follicle. Epidermal cells create the wall of the cysts and secrete the protein keratin inside. This is the thick, yellowish, and odorous substance that may drain from the cyst. These types of cysts should not be confused with sebaceous cysts which are less common and originate from the sebaceous (oil) glands that lubricate the hair and skin.

Potential complications from epidermoid cysts are inflammation that causes the area to become swollen and tender, infection of an abscessed cyst, a rupture that can lead to a boil-like infection, and skin cancer (although extremely rare). In any of the above situations, prompt medical treatment is recommended, but typically a cyst can be left alone. There are some other instances where it is a good idea to have it checked out and possibly removed through minor surgery by a dermatologist. If the cyst appears in an unusual place like the fingers or toes, seems to be growing rapidly, is located somewhere that is irritated consistently by normal movement or clothing, or if it is bothersome to look at, a doctor can guide you through available options. An inflamed cyst is difficult to remove, but an injection may be given to reduce the inflammation and swelling to prepare it for later removal. It is also possible that the doctor will make a small incision and squeeze the fluid out, although this strategy often leads to a return of the cyst later, and should only be performed by a professional.