Kojic Acid for Dark Spots: Does It Work, and Is It Safe?

Kojic acid can fade dark spots, but slowly and modestly. It's a fungus-derived tyrosinase inhibitor that the American Academy of Dermatology lists among over-the-counter ingredients that can fade existing dark spots. Studies support it best for melasma, usually in combination with other lighteners. EU safety experts consider it safe at up to 1%, and allergic reactions, though uncommon, do happen.

Below: how kojic acid works, what the research shows next to the viral kojic-soap claims, the side effects worth knowing, what to do about dark underarms, and a three-way comparison of kojic acid, alpha arbutin and tranexamic acid to help you choose.

What is kojic acid?

Kojic acid is a natural compound made by fungi, used in skincare to slow melanin production. The EU's Scientific Committee on Consumer Safety (SCCS) describes it as a secondary metabolite produced by many species of filamentous fungi, including Aspergillus and Penicillium, and it has long been used in Japan as a food additive to keep produce and seafood fresh. A 2019 review in Biomedicine & Pharmacotherapy notes that it appears in skin creams, lotions, soaps and even dental care products.

It works by blocking tyrosinase, the enzyme that makes melanin. A review of hyperpigmentation in skin of color explains that kojic acid inhibits tyrosinase by chelating (binding) copper at the enzyme's active site, and that it's available in 1% to 4% concentrations, often combined with other lightening agents.

Does kojic acid fade dark spots?

Yes, kojic acid can lighten dark spots, but the evidence is strongest for melasma and for kojic acid combined with other actives, and weaker for it alone. The key studies:

• Kojic acid vs hydroquinone, with glycolic acid: In a split-face study of 39 people with melasma, glycolic acid plus kojic acid on one side and glycolic acid plus hydroquinone on the other gave equal results in 51%, a better result on the kojic side in 28% and on the hydroquinone side in 21%, with no statistically significant difference. The kojic formula was more irritating.
• Kojic acid with hydroquinone: The skin-of-color review notes that studies in white and Asian patients found 2% kojic acid combined with hydroquinone improves melasma.
• Kojic acid alone: A comprehensive melasma review concludes that kojic acid was inferior to hydroquinone in improving melasma scores when each was used alone, and that the supporting studies are small.

For post-acne dark marks, the review of skin of color says clinical studies are still needed to show how well kojic acid works. That doesn't mean it does nothing for those marks; it means the claims run ahead of the data. Expect gradual change over months, and pair it with a complete plan from our guide on how to fade dark spots.

Does kojic acid soap actually work?

Kojic soap may help a little, but the dramatic before-and-after videos are not good evidence, and soap is a weak way to deliver an active ingredient. The kojic acid studies above tested leave-on creams and gels; we did not find controlled trials of kojic soaps in the sources we reviewed. A soap touches your skin briefly before it's rinsed away, while the creams in studies were left on the skin.

Some reasons the viral results can mislead:

• Lighting and filters change how dark skin looks far more than a few days of any product.
• Exfoliation from scrubbing can make skin look briefly brighter without changing pigment.
• Unlabeled strength. The SCCS noted that products are on the market with kojic acid above 1%, the level it considers safe, so a soap's real dose may be unknown.
• Irritation can backfire. The AAD names an irritating product as one trigger for extra melanin in medium and dark skin, so harsh daily scrubbing with a strong soap can leave you with more dark marks.

If you like a kojic soap and it doesn't sting, it's fine to keep using it as a cleanser. Just don't expect it to do the work of a leave-on treatment.

What are the side effects of kojic acid?

The main side effects are irritation and allergic contact dermatitis, and sources disagree on how common the allergy is. The skin-of-color review states that contact dermatitis is a common side effect and that several studies show an increased sensitizing potential. The SCCS, looking at all the human data in its 2022 opinion, concluded that allergic contact dermatitis from kojic acid is very low in occurrence, while still recording case reports, including a woman who developed dermatitis on her face and neck after three days of a product with 0.5% kojic acid.

The other concern is systemic. The SCCS reviewed kojic acid because of reports that it can interfere with how the thyroid takes up iodine, and after weighing potential endocrine-disrupting properties, it judged kojic acid safe as a skin lightener at concentrations up to 1% (opinion SCCS/1637/21, 2022). It also noted that bleaching products applied beyond the face, neck and hands, for example to the arms and décolletage, can raise total exposure.

What this means in practice:

• Choose products at or below 1% when the label states a strength, especially for daily use over large areas.
• Patch test for several days before using kojic acid on your face. In one SCCS case series, reactions in sensitized users appeared 1 to 12 months after they started a product, so watch for new itching or redness even after a good start.
• Stop at the first sign of itching, a rash or worsening darkness, and see a doctor if it doesn't clear.
• If you're pregnant or breastfeeding, ask your doctor before starting any skin-lightening ingredient.

Is kojic acid unstable?

Kojic acid has a reputation for instability, but the published safety data on that point are thin. When the SCCS reviewed it, it recorded that no data were provided on the stability of kojic acid in marketed products. Older melasma research actually described kojic acid as pharmaceutically more stable than hydroquinone. Brands also use oil-soluble derivatives such as kojic dipalmitate; the SCCS opinion did not assess those, so their safety and strength are less documented.

The sensible precautions are the same as for most brighteners: keep the cap closed, store the product away from heat and sunlight, and replace it if it changes color or smell.

Can you use kojic acid on your underarms?

You can, but check why your underarms are dark first, and keep the area small and the product gentle. Dark, thickened skin in body folds can be acanthosis nigricans, which the AAD says can be a sign of prediabetes; it advises seeing a dermatologist if you notice an area of darker, thicker skin. A lightening product won't address that cause.

If a doctor has ruled that out, a few points help:

• Underarms deal with friction, sweat and shaving, so start every other day and build up slowly.
• Don't apply right after shaving, waxing or on broken skin.
• Remember the SCCS point about larger application areas raising exposure; use it on the underarms, not as an all-over body lotion.

Kojic acid vs alpha arbutin vs tranexamic acid: which should you choose?

Choose tranexamic acid first for melasma, alpha arbutin first for low-irritation fading of post-acne marks and sun spots, and kojic acid if you've tolerated it before or want it as part of a combination product. This table compares the three brighteners we've covered:

Lab tests don't settle the ranking either. In a cell-free enzyme test cited in a 2021 review, kojic acid inhibited tyrosinase at a much lower concentration than arbutin, yet in cultured pigment cells arbutin reduced melanin more than kojic acid at the same concentration. For the details on the other two, read our guides to alpha arbutin and tranexamic acid.

Whichever you pick, daily sunscreen matters more than the brightener. The AAD recommends a tinted, broad-spectrum SPF 30+ sunscreen with iron oxide for fading dark spots.

How can Pori help you choose and track a brightener?

Brightening products often mix several actives, plus fragrance or strong acids that can irritate. With Pori Pro, Pori's ingredient scanner reads a product's barcode or label and flags each ingredient it recognizes as beneficial, use with caution or avoid for your skin type, so you can check a kojic serum before it goes on your face. Pori's selfie scan also gives a Pigmentation & Tone reading you can compare over the months a brightener needs.

An app isn't a diagnosis. See a dermatologist for any dark patch that is new, spreading, thickened or changing, and for any spot that bleeds or looks different from the rest.