Medically reviewed by Dr Sin Yong · Last reviewed · 3 min read
Published 13 October 2026 · Reviewed by Dr Sin Yong
Comedone is the medical term for a blocked hair follicle, the starting lesion of acne. Open comedones are blackheads and closed comedones are whiteheads; acne is diagnosed when they recur over weeks and months alongside inflamed spots on the face, chest or back. Unlike a milium, a comedone has an opening, and unlike rosacea's papules, acne has comedones beside its inflamed spots. Inflammation is what turns a comedone into a papule, and what drives both scarring and post-inflammatory pigment change.
Key facts
What it is
A blocked follicle: the non-inflammatory starting lesion of acne
Open vs closed
Open comedones are blackheads; closed comedones are whiteheads; comedonal acne is either without significant inflammation
Versus milium
A comedone has an opening; a milium is a sealed keratin cyst with no punctum, which is why one can be expressed and the other cannot without de-roofing
Treatment
Topical retinoids speed up and normalise the shedding of cells lining the follicle, so existing comedones clear and fewer new blockages form
Where it fits
Acne begins as a comedone and becomes an inflammatory lesion when the follicle wall inflames or ruptures: a papule, a pustule or, at its deepest, a nodule. That sequence is why Dr Sin Yong treats comedones early, because inflammation is the step that produces scars and post-inflammatory marks, and why the presence or absence of comedones separates acne from rosacea and perioral dermatitis. Retinoid purging is the temporary early flare in which micro-comedones already forming come to the surface faster. Full page: acne treatment; the sequence from spot to scar on acne before scars; the sealed look-alike on milia.
Frequently Asked Questions
No. A whitehead is a closed comedone, a blocked follicle that still has an opening to the surface, so it responds to acne treatment. A milium is a sealed keratin cyst with no opening; it does not respond to acne treatment and is removed by a sterile micro-opening in clinic.
The temporary flare some people notice early in a topical retinoid course. Micro-comedones that were already forming come to the surface faster, appearing as small breakouts in the usual areas, before the follicle lining settles into normal shedding.
