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To answer this question in a word: hydroquinone.
A study in the Journal of Cosmetic Dermatology reported kojic acid alone is less efficacious than 2% hydroquinone in treating hyperpigmentation (i.e., sun spots, uneven skin pigmentation, and the like). Kojic acid has also been found to be associated with contact allergy and has a high sensitizing potential (Dermatologic Surgery, 2001).
What about when combined with glycolic acid?
In a review on Medscape, it was reported kojic acid and hydroquinone have the same effect on hyperpigmentation when used together with glycolic acid. However, kojic acid was reported in the same study to be more irritating.
Why Do Any Creams Contain Kojic Acid if Hydroquinone is Better?
Two reasons. First, kojic acid is sometimes preferred because it has greater stability in cosmetic products than hydroquinone (Regulatory Toxicology and Pharmacology, 2001). In fact, it has been affirmed hydroquinone should be stored at low temperature with the cap tightly sealed in order to preserve its effectiveness.
Secondly, kojic acid is sometimes preferred due to recent concern over a possible link between hydroquinone and ochronosis (skin darkening). According to a comprehensive review of 10000 patients, ochronosis is extremely rare, particularly for those patients with lighter skin (Journal of the American Academy of Dermatology, 2007). Still, if you have darker skin tones, you may want to consult with your dermatologist before starting any hydroquinone-based product.
Bottom Line: Use ‘Em Together!
However, hydroquinone and kojic acid inhibit tyrosinase in two different ways (Journal of Pigment Cell Research, 2006). Therefore, scientists have advised using both in conjunction to get the highest degree of tyrosinase inhibition. One I recommend? Age Advantage Laboratories Spot Life Serum ($54.98), with hydroquinone, kojic acid, and glycolic acid.