What Is Kojic Acid? Source, Risks and the 1% Limit in the EU

Its name comes from the koji mould used for fermentation in Japanese cooking, and this molecule has a long history in food too. The reason it was used for years in Japan as an additive to prevent colour change in vegetables, crab and shrimp is also the reason for its use in skin care: it slows the enzymes that drive browning. For how to use it at home and the list of clinical studies, see the arbutin and kojic acid article; this page focuses on the molecule itself and the story of its legal limit.

Where does kojic acid come from, and why does purity matter?

Kojic acid is a secondary metabolite of filamentous fungi, that is, a by-product secreted as moulds grow. It is a crystalline powder from white to pale yellow, with a molecular weight of about 142, and it is water-soluble. In the EU ingredients database it is registered with two functions: whitening and antioxidant.

Because its source is a mould, raw material quality is also a concern. The EU scientific assessment notes that dangerous impurities such as heavy metals and aflatoxin must be kept at trace levels and continuously monitored. As a consumer you cannot measure this, but it is a good reason to stay away from unlabelled "whitening" mixtures with unclear contents.

How does it stop tyrosinase?

Tyrosinase is the enzyme that starts the melanin chain and carries copper in its active site. Kojic acid inhibits this enzyme competitively and reversibly: it gets between the enzyme and its target but does not cause permanent damage. In experiments on mushroom tyrosinase it appears much stronger than arbutin: the concentration that slows the enzyme by half was about 0,12 millimolar for kojic acid and 10 millimolar for arbutin.

This difference does not carry over to skin in the same way. The enzyme in a test tube is alone, while in skin it is behind the barrier. In a safety assessment, black guinea pig skin was lightened with 4% kojic acid but not with 1%. The human data are mixed and small. And note that the 2% and higher concentrations you often see in publications are above the EU limit.

Why does it degrade easily, and are derivatives the answer?

Kojic acid becomes unstable in long storage, which is a known problem for formulators. In response, esters such as kojic acid dipalmitate were developed; they are reported to be more resistant to heat, light and pH changes. But there is a detail here: the EU's 2022 opinion does not cover esters and derivatives such as chloro-kojic acid, because no data were submitted for them. So being "more stable" does not mean it has the same safety data. The same opinion also notes that no report on the stability of kojic acid in market products was submitted. Stop using a product whose colour has changed markedly.

How was the EU 1% limit determined?

This limit did not follow a straight line. Its status changed markedly between the preliminary and final opinions.

The most important data for the change is a study of 6 people that measured blood levels after applying a 1% cream to the face. The amount in the blood was just above the measurement limit. When interpreting it, keep in mind that it is small and single-dose; the committee also noted its limitations.

How serious are the thyroid concern and the allergy?

Thyroid: When given orally to rats, thyroid hormones (T3, T4) fell and TSH, which stimulates them, rose; in rodents, thyroid tumours were seen in the long run. These tumours are explained by disruption of hormone balance rather than direct DNA damage. Humans appear less sensitive to this effect than rodents, but there is no human-specific numerical data for kojic acid. The 1% limit was set taking this uncertainty into account. This does not mean that a 1% cream harms the thyroid in humans; but the question is not fully closed either. If you have a thyroid disease, ask your doctor as a precaution before starting a lightening product.

Allergy: Two pieces of information must sit side by side here. The EU assessment considers the frequency of allergic dermatitis caused by kojic acid to be very low. At the same time, case reports are real: in a 1995 patch test study, sensitisation to kojic acid was found in 5 of 220 female patients, and facial dermatitis appeared 1-12 months after starting the product. In another case, acute dermatitis developed within three days with a product containing 0,5%. So using it for a long time without problems does not mean no reaction will occur later.

When should you see a doctor?

• If, weeks or months after starting the product, itching, swelling or an eczema-like rash appears on the face: stop the product and ask for an evaluation for contact dermatitis.
• If the spot is getting darker where it is applied: do not continue; let the physician plan the treatment.
• If there are widespread, symmetrical spots: it may be melasma; diagnosis and prescription options are in the physician's domain.
• If the spot is growing, bleeding or changing colour: see a dermatologist rather than dealing with it through a care product.
• If you are pregnant or breastfeeding: controlled data are insufficient; do not start on your own.

To try the new product, test it for 7-10 days on the inner arm or elbow crease, applied twice a day. For another ingredient aiming at the same goal, you can look at the arbutin page.