Rosemary leaf extract is among the most potent natural antioxidants in commercial use, and one of the few botanical ingredients used as much for what it does to a formula as for what it does to skin.
The active constituents
Carnosic acid and carnosol are diterpene phenols and the principal antioxidants. Carnosic acid in particular is a powerful inhibitor of lipid oxidation, which is why rosemary extract is widely used to prevent oils and fats from going rancid, in food as well as cosmetics.
Rosmarinic acid is a phenolic acid with documented antioxidant and anti-inflammatory activity, also found in other members of the mint family.
Two roles in one ingredient
Rosemary extract may be present to protect the skin, to protect the formula, or both.
In a product containing significant plant oils, rosemary extract low in the ingredient list is quite likely functioning as a formula stabiliser, preventing those oils from oxidising in the bottle. That is a legitimate and useful role, and it is not the same as an active skin benefit.
Its position in the list is a reasonable guide to which job it is doing.
Extract versus essential oil, which matters
This distinction is important and frequently blurred.
Rosemary leaf extract captures the non-volatile polyphenols: carnosic acid, carnosol, rosmarinic acid. It has little scent and does not trigger fragrance allergen declaration.
Rosemary essential oil, INCI Rosmarinus officinalis leaf oil, is the volatile fraction obtained by distillation. It contains camphor, 1,8-cineole and other terpenes, is strongly aromatic, and carries the irritation and sensitisation considerations that essential oils generally do.
They are different materials with different profiles. A product listing the extract is not using the essential oil, and the caution that attaches to rosemary essential oil, including traditional caution during pregnancy and in epilepsy, relates to the oil rather than the polyphenol extract.
Safety
The extract is well tolerated topically at cosmetic concentrations, with contact allergy uncommon.
Anyone with a known allergy to the Lamiaceae family, which includes mint, basil, sage, lavender and oregano, may cross-react.
