The cytotoxic effects of HQ are not limited to melanocytes, although the dose required to inhibit cellular metabolism is much higher for nonmelanotic cells than for melanocytes. Thus, HQ can be considered a potent melanocyte cytotoxic agent with relatively high melanocyte-specific cytotoxicity. HQ is also a poor substrate of tyrosinase, thereby competing for tyrosine oxidation in active melanocytes. However, for better efficacy, it often is compounded into various mixtures for treatment of hyperpigmentation. Despite its remarkable overall safety, the physician ought to bear in mind potential side effects.
Contact dermatitis occurs in a small number of patients and responds promptly to topical steroids.
An uncommon, yet important, side effect of HQ is exogenous ochronosis. This disorder is characterized by progressive darkening of the area to which the cream containing HQ is applied. Histologically, degeneration of collagen and elastic fibers occurs. Exogenous ochronosis generally has been observed in black patients and after use of high concentrations of HQ for many years. A South African epidemic of exogenous ochronosis due to HQ has been reported.