Strong inhibitor of melanin (skin pigment) production that has long been established as the most effective ingredient for reducing and potentially eliminating brown spots and hyperpigmentation from melasma, because it prevents skin from producing melanin, 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 limit 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 the primary topical ingredient for inhibiting melanin production. Using it in combination with other ingredients—especially tretinoin—can greatly reduce and even eliminate skin discolorations. [2,3]
Some concerns about hydroquinone’s safety on skin have been expressed, but the research regarding topical application indicates that negative reactions are minor, are the result of using extremely high concentrations, or result from using it in combination with other agents such as glucocorticoids or mercury iodine. This is particularly true in Africa, where adulterated skin-lightening products are commonplace. 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 skincare products.” Maibach also stated that “hydroquinone is undoubtedly the most active and safest skin-depigmenting substance....” 
Despite the controversy, abundant research from reputable sources shows hydroquinone to be safe and effective. Surprisingly, there is even research showing that workers who handle pure hydroquinone actually have a lower incidence of cancer than the population as a whole. [5,6,7,8]
You may have read that hydroquinone is linked to leukemia. That connection has to do with the chemical benzene, which can be metabolized into hydroquinone. It’s the benzene as the starting point that raises that concern, it’s not the hydroquinone as used in topical skin-lightening products. Other research examining this connection was carried out on cultured internal (inside the body) cells or via oral consumption of pure hydroquinone, which is not at all related to how hydroquinone is used in skincare products. More to the point, the amount of hydroquinone used in the studies that suggest an association with leukemia are considerably higher amounts than what people are exposed to via skincare products that contain hydroquinone. Research also 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. [5,6,7,8,9] Bottom line: 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. If you’re struggling with brown spots or sun-induced skin discolorations, hydroquinone remains among the best ingredient to treat them. [5,6,7,8,9]
- Yoshimura K, Tsukamoto K, Okazaki M, Virador V, Lei T, Suzuki Y, Uchida G, Kitano Y, Harii K. Effects of all-trans retinoic acid on melanogenesis in pigmented skin equivalents and monolayer culture of melanocytes. J Dermatol Sci.. 2001;27(S1):S68-75.
- Grimes P, Kelly A, Torok H, Willis I. Community-based trial of a triple-combination agent for the treatment of facial melasma. Cutis. 2006;77(3):177-84.
- Fabi S, Goldman M. Comparative Study of Hydroquinone-Free and Hydroquinone-Based Hyperpigmentation Regimens in Treating Facial Hyperpigmentation and Photoaging. J Drugs Dermatol. 2013;12(3):S32-7.
- Wester R, Melendres J, Hui X, Cox R, Serranzana S, Zhai H, Quan D, Maibach H. Human in vivo and in vitro hydroquinone topical bioavailability, metabolism, and disposition. J Toxicol Environ Health A. 1998;54(4):301-17.
- Draelos Z. Skin lightening preparations and the hydroquinone controversy. Dermatol Ther. 2007;20(5):308-13.
- Tse T. Hydroquinone for skin lightening: safety profile, duration of use and when should we stop? J Dermatolog Treat. 2010;21(5):272-5.
- DeCaprio A. The toxicology of hydroquinone--relevance to occupational and environmental exposure. Crit Rev Toxicol. 1999;29(3):283-330.
- McGregor D. Hydroquinone: an evaluation of the human risks from its carcinogenic and mutagenic properties. Crit Rev Toxicol. 2007;37(10):887-914.
- Levitt J. The safety of hydroquinone: a dermatologist’s response to the 2006 Federal Register. J Am Acad Dermatol. 2007;57(5):854-72.