Strong inhibitor of melanin production that has long been established as the most effective ingredient for reducing and potentially eliminating brown spots and hyperpigmentation from melasma (Source: Journal of Dermatological Science, August, 2001, Supplemental, pages 68–75), meaning that it prevents skin from making the substance responsible for skin color. Hydroquinone does not bleach the skin, which is why "bleaching agent" is a misnomer; it can't remove pigment from the skin cell but it can control excess melanin production. Over-the-counter hydroquinone products can contain 0.5% to 2% concentrations of hydroquinone; 4% (and sometimes higher) concentrations are available only from physicians.
In medical literature, hydroquinone is considered the primary topical ingredient for inhibiting melanin production. Using it in combination with some other ingredients—especially tretinoin—can greatly reduce and even eliminate skin discolorations (Sources: Cutis, March 2006, pages 177–184; Journal of Drugs in Dermatology, September–October 2005, pages 592–597; Journal of Cosmetic Science, May–June 1998, pages 208–290; and Dermatological Surgery, May 1996, pages 443–447). Interestingly, hydroquinone also is a potent antioxidant (Source: Journal of Natural Products, November 2002, pages 1605–1611).
Some concerns about hydroquinone's safety on skin have been expressed, but the research when it comes to topical application indicates that negative reactions are minor, are a result of using extremely high concentrations, or result from the use of other skin-lightening agents such as glucocorticoids or mercury iodine. This is particularly true in Africa, where adulterated skin-lightening products are commonplace (Sources: British Journal of Dermatology, March 2003, pages 493–500; and Critical Reviews in Toxicology, May 1999, pages 283–330).
According to Howard I. Maibach, M.D., professor of dermatology at the University of California School of Medicine, San Francisco, "Overall, adverse events reported with the use of hydroquinone ... have been relatively few and minor in nature.... To date there is no evidence of adverse systemic reactions following the use of hydroquinone, and it has been around for over 30 years in skin-care products." Maibach also stated that "hydroquinone is undoubtedly the most active and safest skin-depigmenting substance...." Research supporting Maibach's contentions was published in the Journal of Toxicology and Environmental Health (1998, pages 301–317). Concern about hydroquinone having carcinogenic properties is mostly related to industrial-grade materials and uses. For cosmetic use there appears to be no similar evidence.
Despite hydroquinone’s impressive track record and efficacy, the FDA, in September 2006, recommended that products containing hydroquinone be sold only with a prescription due to their opinion that it posed certain health risks. The FDA asserts there are animal studies showing it may be a possible carcinogen, and studies from Africa showing there is a risk of a skin disorder called ochronosis (Source: http://www.fda.gov/OHRMS/DOCKETS/98fr/E6-14263.htm).
However, there is abundant research from reputable sources that shows hydroquinone to be safe and extremely effective (Sources: Cutis, August 2006, Supplemental, pages 6–19; Journal of Cosmetic Laser Therapy, September 2006, pages 121–127; American Journal of Clinical Dermatology, July 2006, pages 223–230; and Journal of the American Academy of Dermatology, May 2006, Supplemental, pages 272–281). Surprisingly, there is even research showing that workers who handle pure hydroquinone actually have lower incidences of cancer than the population as a whole (Source: Critical Reviews in Toxicology, May 1999, pages 283–330).
As for hydroquinone being linked to leukemia, the link has to do with the chemical benzene, which can be metabolized into hydroquinone. It’s the benzene as the starting point that’s cause for concern, not hydroquinone as it is used in topical skin lightening products. Other research examining this connection was done on cultured internal cells or via oral consumption of pure hydroquinone, which is not related to how hydroquinone is used in skin-care products.
More to the point, the amounts of hydroquinone used in studies that suggest an association with leukemia are considerably larger than what people are exposed to via skin-care products that contain hydroquinone. Moreover, research has shown that humans metabolize hydroquinone completely whereas rats (the most common animal used in toxicity studies for hydroquinone) metabolize hydroquinone much differently, which is likely why oral administration or injection of hydroquinone causes the problems noted in the research. It has been shown that systemic exposure to hydroquinone from "routine topical application" is "no greater than from quantities present in common foods". That's right: many commonly consumed foods contain trace amounts of hydroquinone, including wheat products, coffee, herbal teas, pears, broccoli, and berries (Sources: Leukemia, May 2010, pages 986–991; Toxicology and Applied Pharmacology, May 2010, pages 273–279; Journal of the American Academy of Dermatology, November 2007, pages 854–872; Journal of the European Academy of Dermatology and Venereology, Volume 20, 2006, page 781; and Journal of Toxicology and Environmental Health, January 1996, pages 31–46).
Finally, despite what you may have read, hydroquinone is not carcinogenic (cancer-causing). Considerable analysis of the animal research that raised this concern has shown that hydroquinone is not and cannot be classified as a human carcinogen (Source: Critical Reviews in Toxicology, Volume 10, 2007, pages 887-914). If you're struggling with brown spots or sun-induced skin discolorations, hydroquinone remains the best ingredient to treat them.