What Causes White Horizontal Lines on Teeth After Bleaching? Can They Be Removed?

Usually those white lines (decalcification areas) were present prior to the bleaching, but may not have been noticeable. The bleaching makes them even whiter, as it makes the rest of the tooth whiter as well. It will often begin to blend in again after a few days.

As you whiten your teeth, the bleach dehydrates them causing all kinds of internal character to show up.  Pay no attention to this as it has always been there, just not noticeable when the teeth are fully hydrated.  Continue with the regimen to get the best whitening results an rest assured that when you complete treatment, these inconsistencies will again blend in but at a much lighter shade. 

Good Luck!