Hydroquinone is known to inhibit melanogenesis both in vitro and in vivo. In this study, 2% and 5% hydroquinone creams were topically applied to the hyperpigmented skin of 56 patients. Used in this way, hydroquinone was a moderately effective depigmenting agent in 80% of cases. The 2% cream appeared to be as effective therapeutically as the 5% cream and to evoke untoward side effects (primary irritation) much less often. Therapy with topically applied hydroquinone did not lead to complete disappearance of pathological hypermelanosis, but results were satisfactory enough to help most patients become less self-conscious about their pigmentary abnormalities.
Arndt KA, Fitzpatrick TB. Topical Use of Hydroquinone as a Depigmenting Agent. JAMA. 1965;194(9):965-967. doi:10.1001/jama.1965.03090220021006