The most popular ingredient in lightening creams, hydroquinone was banned by the EU because of deadly chemicals such as mercury and hydroquinone which are said to have caused cancer and liver problems in rats.
Even other lightening substances prescribed by dermatologists seem to be deadly if used for too long.
What’s the reason for this?
The short version, because the chemical reactions they perform in skin cells affect DNA. If DNA changes, two things can happen: the cell dies (which isn’t good for the cell, but unless it happens massively doesn’t kill the rest of the body) or the tumor starts growing.
The longer version can take several books’ worth, depending on how much chemistry you already know.