It is a tyrosinase inhibitor. Tyrpsinase is an enzyme vital to tthe production of melanin. Less Tyrosinase=less melanin. At 10%, I do find it does increase exfoliation but I don't know if this is a typical experience or an anomaly peculiar to my skin. It must be continued in order to maintain results but not 2x/day. Most people tolerate it well but like with anything, some folks cannot use it. There are several links throughout this forum re HQ. If you check the HQ cancer urban legend thread, there is more info as well as important safety info for compounders.