Kojic acid is a substance derived by funghi—namely, mushrooms, according to Dr. Graf. According to Dr. Shirazi, the sweet spot for kojic acid in a serum is 1 to 2%. Dr. Greenfield also advises against using harsh acids or prescription retinoids alongside kojic acid, as they can further irritate the skin. To minimize the risk of adverse reactions, Dobos recommends performing a patch test before introducing kojic acid into your routine. Also, if you regularly use a retinoid, alternate usage of the two—retinol one night, kojic acid on the other.