Butyl Stearate

Butyl Stearate is basically stearic acid (a fatty acid from plant or animal fats) joined to butyl alcohol. In cosmetics, it's a lightweight emollient that gives products a smooth, silky slip.

It has a pretty unremarkable safety profile: the Cosmetic Ingredient Review Panel found it minimally irritating to skin at most but non-sensitizing, non-phototoxic, and non-photosensitizing. A true allergy to it is rare with no sensitization signal from repeat-insult patch testing at 50%.

Formulas usually use this ingredient between 1-5%.

Is it comedogenic?

Fulton's rabbit-ear work specifically named Butyl Stearate among the comedogenic "offenders" alongside Isopropyl Myristate and its analogs. It's just worth remembering the authors themselves called the rabbit ear "not an ideal animal model but the best we have".

This is because testing ingredients for comedogenicity on rabbit ears means it over-predicts because rabbit ears are more sensitive than human skin. And comedogenic tests use the raw material, not a small percentage blended into a cosmetic formula.

Any finished product with this ingredient won't automatically make you break out; it really comes down to concentration, formula, and your skin.

Is it fungal acne safe?

This ingredient may not be fungal acne, or Malassezia safe. This is because the Malassezia yeast breaks down surrounding lipids for fatty acids it likes. Butyl Stearate contains stearic acid (C18) which is in the chain-length range the yeast can feed on (~C11-24). Just know this ingredient has never been tested against Malassezia, so this is just a precaution for people managing it.