Comedogenicity
Comedogenicity is the tendency of an ingredient or finished product to induce comedones by driving follicular hyperkeratinisation, and it is distinct from acnegenicity.
Comedogenicity describes a specific process: retention hyperkeratosis inside the follicular infundibulum, where corneocytes fail to shed and accumulate into a microcomedo that can become an open or closed comedone. It develops over weeks. Acnegenicity is faster and different: a substance provokes inflammatory papules and pustules within days, often through irritation rather than follicular plugging. The two are routinely merged in product copy.
The classic screen is the rabbit ear assay of Kligman and Mills, in which a substance is applied repeatedly to the rabbit external ear canal and follicular keratin accumulation is scored histologically. The human version uses the upper back, applied under occlusion for around four weeks, with cyanoacrylate follicular biopsies counting microcomedones; isopropyl myristate serves as the standard positive control. The rabbit ear is far more sensitive than human skin and flags many ingredients that behave acceptably on a human face.
The decision this changes is which layer of information you are reading. A comedogenicity result on a neat raw ingredient predicts almost nothing about a finished formulation containing it at low percentage in a different vehicle. Concentration, occlusivity of the base, and the rest of the ingredient list all shift the result.
The frequent error is treating the circulating zero-to-five comedogenic rating lists as authoritative product guidance when they derive largely from undiluted ingredients on rabbit ears. The other is reading the phrase non-comedogenic as a certification; in the United States it is not defined in regulation, requires no particular test, and is not verified before marketing. For peptide serums, the peptide is rarely the culprit when breakouts follow; the vehicle carries the occlusive load.