Sebaceous (epidermoid) cysts often produce a foul, cheesy odor, particularly when infected or ruptured. This smell arises from the breakdown of trapped sebum and keratin—a thick, white, cottage-cheese-like protein material—inside the cyst. The smell is a sign of decomposition, bacterial presence, or infection.
Wikipedia
Wikipedia
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Key Details About Cyst Odors
What it smells like: The smell is often described as cheesy, like rotten eggs, stale urine, or a "broken, spoiled can of cream of chicken soup".
When it happens: The odor is most intense if the cyst ruptures, drains, or becomes infected, inflamed, and tender.
Causes of odor: The buildup of dead skin cells (keratin) and oily sebum produces a pungent smell, which worsens when bacteria colonize the area.
When to see a doctor: If a cyst is releasing a foul-smelling, thick substance, or if it is painful, swollen, and red, it is likely infected and warrants professional care.
PubMed Central (PMC) (.gov)
PubMed Central (PMC) (.gov)
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This is my guess what's going on with your back... I have a spot on my chest that does this occasionally... The odor when popped is awful.