Hydroquinone and mercury are ingredients added to skin-lightening products to limit the skin's production of melanin. Melanin is the pigment responsible for skin colour. It is used to lighten the dark patches of skin (also called hyperpigmentation, melasma, "liver spots," "age spots," freckles) caused by pregnancy, birth control pills, hormone medicine, or injury to the skin. This medicine works by blocking the process in the skin that leads to discoloration.

When people apply skin products with Hydroquinone or mercury to the skin, the body absorbs these ingredients. With continued use, Hydroquinone and mercury can build up in your body, which may cause harm.

In addition, Hydroquinone is a potent ingredient used in prescription medicines to treat skin conditions. The ingredient is prohibited in skincare cosmetic products and should only be used under medical supervision, as Hydroquinone may cause harmful effects if not used properly. The inappropriate use of Hydroquinone could result in changes in skin colour and hypersensitivity reactions. The FDA has received reports of serious side effects from the use of Hydroquinone, including rashes, facial swelling, and permanent skin discoloration (ochronosis).

More importantly, skin-lightening products containing Hydroquinone are not approved for over-the-counter sale. However, humans can get a prescription skin product containing Hydroquinone from healthcare providers. The use of mercury in skin-lightening products is banned.