Hello and welcome to healthcaremagic Hydroquinone is a skin lightening agent. It inhibits the enzyme tyrosinase, which is important in the synthesis of new melanin. Although it can be used safely in lower concentrations like 2% for a longer period of time but higher strengths like 4% or 5% should not be used for longer duration of time because it can cause a accumulation of ochronotic pigment in the skin, called as exogenous ochronosis, which is permanent Yo could rather use kojic acid 2% which is also a skin lightening agent but devoid of this side effect and can therefore be used for a longer duration unlike hydroquinone. regards