What is kojic acid and what does it do?

What is kojic acid?

Kojic acid is a small organic molecule that some fungi produce during fermentation. In skincare it is mostly found in brightening serums, creams and soaps. It is a popular choice for people looking for an alternative to stronger, prescription dark spot treatments such as hydroquinone.

Pure kojic acid breaks down easily and turns brown when exposed to light, heat and air. That is why some products use a more stable derivative, kojic dipalmitate; however, there is less data on how well this derivative works in the skin than on pure kojic acid.

How does it work on dark spots?

Melanin, the brown pigment in skin, is made with the help of an enzyme called tyrosinase. Kojic acid binds the copper this enzyme needs to work, reducing its activity and so slowing the production of new pigment. It does not erase existing spots; as new pigment decreases and the skin renews itself, spots gradually fade.

Seeing results takes patience: for most people the difference becomes clear after 8–12 weeks of consistent use. Kojic acid can work better alongside ingredients that act in other ways, such as glycolic acid, niacinamide or vitamin C.

How to use it

After cleansing, apply a thin layer of kojic acid serum or cream to the dark spots or the whole face, then follow with your moisturizer. On sensitive skin, start with a few evenings a week and move to every evening if you tolerate it well after two or three weeks. Applying it evenly around the spots, instead of piling product onto one point, avoids a lighter ring forming around the spot.

While using kojic acid, a sunscreen with SPF 30–50 and high UVA protection every morning is essential. No brightening ingredient gives lasting results without sun protection; for melasma in particular, tinted sunscreens with iron oxides also protect against visible light.

If your kojic acid product has turned deep yellow or brown over time, it may have degraded. Store it somewhere cool and dark and close the cap tightly.

Does kojic acid soap work?

Kojic acid soaps are popular, especially for dark spots on the body. But soap stays on the skin for only a few seconds before being rinsed off; that brief contact greatly reduces the effect compared with a leave-on serum. The alkaline nature of bar soap and the high kojic acid content of some products can also cause dryness and irritation.

For dark spots on the face, leave-on serums or creams make more sense. If you use a soap, it is safer on the body rather than the face, used briefly and followed by moisturizer.

Side effects and precautions

The most common side effects of kojic acid are redness, itching, stinging and dryness. It is more likely than many other brightening ingredients to cause contact dermatitis (allergic or irritant skin inflammation), so testing it for a few days on a small area, such as behind the ear or under the jaw, before first use is a good habit.

The European Union's Scientific Committee on Consumer Safety (SCCS) found kojic acid safe at up to 1% in face and hand products. Data on use during pregnancy and breastfeeding are limited; during this time, options with more data, such as azelaic acid, are usually preferred for dark spots, so talk to your doctor before deciding.