how effective are whitening products for you?

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we see alot of commercially sold beauty products nowadays claiming to whiten the skin in 1 week or so. in fact if you will conduct a survey regarding the most frequently purchased products you will see that whitening products are the one that generate the highest sales making their manufacturers richer and richer by millions of dollars. in tv ads you will see pretty faces of commercial models and actresses endorsing a certain brand of whitening products and testifying that it has whiten their skin, making the viewers believe that it was the real product who made them what they look like today. i wonder how good these products are, do they really whiten the skin in 7 days or so? if that's the case how come there are still people who don't have a fair skin? if whitening products really whiten the skin in 2 weeks how come people are buying it for more that a year and still nothing has change on their skin tone. what do you think? are they really effective or just a waste of money?