Sebaceous cyst on an 89 year old man's shoulder. Cysts are pus-filled swellings, that if left, can burst and release their contents. They are most painful just prior to the discharge of the pus. Sebaceous cysts form when the secretory (sebaceous) glands associated with hair follicles become blocked. The formation of the cyst is harmless, but any infection needs treating. Treatment is with surgical drainage of the cyst and a course of antibiotics.