Whitening agents sold in pharmacies ,existing in the market ,being bought by the  patients, and being used by  the physician is effective only  within enamel. (enamel is layer that surrounds the tooth from the top to bottom.)

If you apply a bleaching agent, under the enamel, to  sensitive layer –dentin- ,as it is very close to the nerves you may have very serious problems.

This layer  may occur when tooth  layer  is worn out ,  and in other words enamel may not be existing in the the patient's teeth . Patients may not recognize the structure of enamel and dentin, and usage of bleaching agents bought from  pharmacies  may cause of  arising of  very serious problems . In the same way if this application have been made by a physician it may cause serious health problems . Such problems may lead to treatment of patients experiencing treatment process channel.

If you have had teeth discoloration, it will not been cleared without  self-harm . But the risk of patients in coloring  can be eliminated;

By Marvellous brushing ,
By not using caffeinated beverages,

By not using tobacco,etc
By standing  away from solutions found in drinking water  such as excessive chlorine, fluorine, and so on.
You can have healthy and white teeth.

But under the supervision of a professional treatment should be started after the formation of pigmentation.