Concentrations vary from a 2% concentration available in the United States without a prescription to a standard 4% concentration and even higher when compounded. Efficacy is directly linked to concentration, but the incidence of adverse effects also increases with concentration. All concentrations can lead to skin irritation, phototoxic reactions with secondary postinflammatory hyperpigmentation, and irreversible exogenous ochronosis (reported even with long-term use of 2% HQ). Special care must be taken not to prescribe the monobenzyl ether of HQ (Benoquin), which causes an irreversible localized and generalized vitiligolike leukoderma. Outside the United States, topical creams with concentrations as high as 8% are available over the counter. These agents are associated with much higher rates of exogenous ochronosis and should not be used.