Hello, my name is Dr.Dorina Gurabardhi. I understand your concern. 
Skin lightening products work by reducing a pigment called 
melanin in the skin.
How much melanin your skin has is mainly a matter of your genetic makeup.
- Sunlight exposure,
- hormones, 
-skin damage, and 
-exposure to certain chemicals can also affect melanin production.
Over-the-counter skin lighteners can contain up to 2% 
hydroquinone. Dermatologists can write prescriptions for lighteners that contain 4%-6% hydroquinone.
Some advices that I can give to you are:
-Don't use it for to long-term 
-Always use sunscreen when using a skin lightener and going out in the sun.
-Make sure an over-the-counter skin lightener with hydroquinone has no more than 2% of that chemical.
-Make sure it has no mercury in it
Hope my answer was helpful, if you have any other questions I'll be glad to help, thanks!