Hydroquinone

Hydroquinone is a potent depigmenting active typically used at 2–4% (and higher in compounded formulations), and irritant contact dermatitis (burning, stinging, erythema, scaling) is a well-documented, concentration- and vehicle-dependent adverse effect in clinical use. In sensitive or eczematous skin, barrier disruption markedly increases the likelihood of irritation, and cumulative irritation is common when combined with retinoids, acids, or benzoyl peroxide in routine regimens. Given its frequent need for careful introduction and the meaningful rate of intolerance in reactive patients, it warrants a high irritancy score. Safety Notes: In current consumer OTC skincare markets where hydroquinone is permitted, the lowest observed levels are typically ~0.25–0.5% in low-strength brightening creams/serums designed for gradual tone evening. The highest widely available non-prescription consumer products are 2% hydroquinone in leave-on spot treatments/creams, reflecting the common OTC cap in markets that still allow it (higher strengths such as 4% are generally prescription-only and excluded). Hydroquinone is primarily used in leave-on products; rinse-off usage is uncommon due to reduced contact time and efficacy.