A white crystalline phenol (phenol is a mildly acidic toxic white crystalline solid obtained from coal tar and used in chemical manufacture) that is used as a photographic developer, an antioxidant and stabilizer, and to remove pigmentation from hyperpigmented areas of skin.
Hydroquinone is found in products used to lighten/brighten pigment. It is a metabolite of benzene, which is linked to cancer. It may cause skin irritation and possibly encourage the discoloration it’s meant to correct.
While hydroquinone is currently legal in the US in over-the-counter cosmetics, it is banned in several other countries including the UK and Canada.
 NIH. NCBI. Exogenous Ochronosis. Prachi A Bhattar, Vijay P Zawar,1 Kiran V Godse, Sharmila P Patil, Nitin J Nadkarni, and Manjyot M Gautam. 2015. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4681189
 Government of the Netherlands. Ban on hydroquinone in Lightening Creams. (2001). http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.538.8859&rep=rep1&type=pdf
 BBC. Skin Whitening Creams. Ashni Lakhani & Elise Wicker. 2018. https://www.bbc.com/news/uk-45085674
 The Government of Canada. “Hotlist”, ingredients that have been restricted for cosmetic products. (2019).