Kojic acid, derived from mushrooms and other plant materials, is a common ingredient in skin lightening/fading products, face and body moisturizers, anti-aging creams and lotions, around-eye creams, facial cleansers, sunscreens and other skin care products. Its main purpose, according to KojicAcid.org, is to treat hyperpigmentation, which occurs when areas of the skin develop too much melanin. Melanin is the substance that gives skin color, such as moles, age spots and freckles. Kojic acid can cause some side effects, particularly for those with sensitive skin.
Skin irritation is the most common kojic acid side effect, according to the online Materials Safety Data Sheet (MSDS), which goes on to note that in its pure form kojic acid, "May be harmful by inhalation, ingestion and if absorbed through skin." However, the amount of kojic acid in skincare products is usually not enough to trigger irritation, unless the user has especially sensitive skin, and some kojic acid products contain topical corticosteroids to counter any potential irritation.
Allergic Contact Dermatitis
Even if kojic acid does not initially irritate the skin, KojicAcid.org notes that there's a danger of developing allergic contact dermatitis with continued regular use. This skin condition is characterized by a red rash or bumps, itching, pain, blisters and dry, red patches of skin--the same reaction one might get from contact with poison ivy or poison oak. Discontinue use if you note these side effects.
Other Side Effects
In a review of 165 kojic acid toxicity studies, CosmeticsDatabase.com, run by the nonprofit Environmental Working Group (EWG), notes that some studies have indicated that the substance causes cell mutation in mammals, but that it is not likely to be a human carcinogen. Some animal studies have shown high amounts of kojic acid to cause liver, kidney, reproductive, cardiovascular, gastrointestinal and respiratory side effects, and high doses have been shown to be toxic to the brain and nervous system in animal studies. The EWG notes that the cosmetics industry has not formed a panel to assess kojic acid's safety in humans, and that further research is needed.