Kojic acid soap for hyperpigmentation - Kojic acid is a popular ingredient in many skincare products because it is effective in lightening the skin and reducing hyperpigmentation. Kojic acid soap is an especially popular product for those looking to treat hyperpigmentation because it is affordable and easy to use. Simply lather the soap on the affected area and rinse it off with water. You will likely see a difference in the appearance of your skin after a few weeks of regular use.