Kojic acid (5-hydoxy-4-pyran-4-one-2-methyl) is a fungal metabolic productand inhibits the catecholase activity of tyrosinase, which is the rate-limiting, essential enzyme in the biosynthesis of the skin pigment melanin. Kojic acid also is consumed widely in the Japanese diet with the belief that it is of benefit to health. Indeed, it has been shown to significantly enhance neutrophil phagocytosis and lymphocyte proliferation stimulated by phytohemagglutinin. Melanocytes treated with kojic acid become nondendritic with a decreased melanin content.
Additionally, it scavenges reactive oxygen species that are released excessively from cells or generated in tissue or blood.
Kojic acid is used in concentrations ranging from 1-4%. Although effective as a skin-lightening gel, it has been reported to have high-sensitizing potential and cause irritant contact dermatitis. In a study comparing glycolic acid/kojic acid combination with glycolic acid/HQ, no statistical difference in efficacy existed between kojic acid and HQ. However, the kojic acid preparation was reported to be more irritating.