Kojic acid

What it is

Kojic acid is a metabolic product of fungi such as Aspergillus³, and a by-product of fermenting rice for sake and soy sauce. It lightens skin by inhibiting tyrosinase, the enzyme that starts melanin production¹. In skin care it appears at around 1 to 4% in serums, creams and soaps⁴, often paired with glycolic acid or vitamin C¹. It is unstable in light and heat, which is why products often turn brown as they age.

What the evidence says

Evidence: limited. Small comparative studies show kojic acid lightens melasma, almost always in combination products and more slowly than hydroquinone; there are no large trials of kojic acid on its own.

In a 60-person study, a 0.75% kojic acid cream (with 2.5% vitamin C) improved melasma by week 8 and clearly by week 12, but 4% hydroquinone worked faster and better at every visit². A 2018 systematic review found mixed results across the handful of kojic acid studies: one combination with glycolic acid matched hydroquinone, another found 1% kojic acid most useful when added to hydroquinone, and the authors called for research on kojic acid alone¹. Its main problem is not weakness but skin reactions. In a Japanese patch-test series, five of eight people who had used 1% kojic acid products were allergic to it, with facial dermatitis beginning one to twelve months after they started³. For the full picture on pigment, read dark spots and hyperpigmentation.

How people use it

• A serum or cream at 1% or less, once daily on dark patches, moving to twice daily only if the skin stays calm.
• As part of a blend with gentler brighteners such as niacinamide or azelaic acid rather than as the only active.
• With broad-spectrum sunscreen every morning, because the marks it targets darken with sun exposure.
• In courses of about three months with breaks, rather than year-round.

Cautions

• Contact allergy is relatively common^{3, 4}. Patch test on the inner arm twice a day for seven to ten days, and stop if you develop itching, redness or scaling on the face.
• Safety limits: the EU's scientific committee concluded in 2022 that kojic acid is safe as a skin-lightening agent up to 1%, after weighing its possible endocrine effects, and noted that use with peeling agents increases absorption⁵. Keep it off freshly peeled or broken skin.
• Irritation itself triggers new dark marks on brown and Black skin, so introduce it slowly; see melanin-rich skin care.
• There is no safety data in pregnancy, so ask your clinician first. If a mark is changing, raised or bleeding, see when to see a dermatologist.