Kojic acid soap is a type of skin-lightening or skin-brightening product that contains kojic acid as its active ingredient. Kojic acid is a natural substance derived from various species of fungi, and it's often used in cosmetics and skincare products for its ability to lighten skin and reduce the appearance of dark spots, age spots, and other pigmentation issues.

Kojic acid works by inhibiting the production of melanin in the skin. Melanin is the pigment that gives skin its color, and an overproduction of melanin can lead to hyperpigmentation. By inhibiting melanin production, kojic acid can help even out skin tone and reduce the appearance of dark spots.

It's important to note that while kojic acid can be effective for some people, it may cause skin irritation or allergic reactions in others, especially those with sensitive skin. Additionally, prolonged use of kojic acid products may make the skin more sensitive to sunlight, so it's important to use sunscreen when using these products.

As with any skincare product, it's a good idea to do a patch test before using kojic acid soap to check for any adverse reactions, and it's best to consult with a dermatologist or skincare professional, especially if you have concerns about skin sensitivity or existing skin conditions.