2- Hydroquinone:
Its bleaching properties were discovered when it was observed that colored tanners wearing rubber gloves acquired discolored areas on hands and forehands. The studies on the cause of this effect pointed out to hydroquinone, an agent used in rubber synthesis (Parvez, 2007). Hydroquinone is a hydroxyphenolic compound that inhibits the synthesis of melanin by inhibiting tyrosinase enzyme It may also function by interfering with the formation or degradation of melanosomes and by inhibiting the synthesis of DNA and RNA within melanocytes.
Hydroquinone is a most widely used depigmenting agent at present, but is considered to be highly cytotoxic to melanocytes and potentiality mutagenic to mammalian cells (Thiele et al., 1997; Donsing and Viyoch, 2008). It can be irritating and causes redness and burning, also it has been shown to cause exogenous ochronosis. Ochronosis may result in loss of elasticity of the skin and impaired wound healing which has resulted in a ban on its over-the-counter use in USA and many other countries. It was only ever allowed for small areas of skin and for treating things like age spots or sun spots (Gabriel, 2008).