The skin condition caused by prolonged use of hydroquinone creams is ochronosis, specifically exogenous ochronosis.
Unlike ordinary hyperpigmentation or melasma, exogenous ochronosis involves the buildup of dark pigment in connective-tissue structures of the skin. It can produce blue-black or brown-grey discoloration, often in areas repeatedly treated with hydroquinone. The condition may also develop after exposure to other phenolic compounds.
Ochronosis has two broad forms. Endogenous ochronosis is associated with the inherited metabolic disorder alkaptonuria, while exogenous ochronosis is acquired through topical or environmental exposure. Hydroquinone-induced cases were described in patients using skin-lightening creams in the 1970s, although an earlier phenol-related case was reported in 1912.
A common mix-up is calling the condition simply melasma, because hydroquinone is often used to treat melasma in the first place. Exogenous ochronosis is uncommon but can be difficult to reverse, so prolonged or unsupervised use of hydroquinone is a major warning sign.