The most popular combination consists of HQ, a topical steroid, and retinoic acid. Prolonged HQ usage may lead to untoward effects like depigmentation and exogenous ochronosis. Hydroquinone (HQ), also known as dihydroxybenzene, is a hydroxyphenolic compound that is structurally similar to precursors of melanin. It inhibits the conversion of DOPA to melanin by inhibition of the enzyme, tyrosinase.. On the other hand Retinoids, such as tretinoin, were first used in combination with HQ as penetration enhancers, but were later recognized to have their own effect on melanogenesis.retinoids affect multiple steps in the melanization pathway. Tretinoin promotes the rapid loss of pigment through epidermopoiesis and increased epidermal turnover decreases the contact time between keratinocytes and melanocytes.[8] Retinoic acid (RA) suppresses UVB-induced pigmentation by reducing tyrosinase activity. The acid acts at a posttranscriptional level on tyrosinase and tyrosinase-related protein.compared with phenolic compounds like HQ, RA takes a much longer time to act; clinically significant lightening becomes.evident after 24 weeks. retinoin monotherapy has produced a good therapeutic response in clinical trials but better results are obtained in combination with other agents like HQ and corticosteroids. The most common side effects include erythema, burning, stinging, dryness, and scaling.so better u buy a cream with trade name isotrex..