Knowing that the bleaching gel works on the enamel proteins, teeth whitening is only effective on natural enamel. Therefore, there are some limitations to the treatment:

By natural anatomy, the enamel gets thinner near the gum. The dentine, a yellow bone layer beneath the thin layer of enamel is visible by transparency. It is important to understand this natural aspect and not trying to bleach the lower third of the teeth. Doing so, you also avoid all gum irritation problem during the treatment.

The fillings, silver or white, do not contain any enamel to be whiten. Therefore the shade of the fillings will remain the same after the treatment.  They can be replaced after the bleaching if they compromise the brightness of your smile.