Internal teeth bleaching is used to lighten a discolored tooth that has had root canal therapy

 It involves placing a chemical oxidizing agent within the coronal portion of a tooth to remove discoloration.  The etiology of tooth discoloration can be intrinsic, extrinsic, or both; it can involve dentin, enamel, or pulp; it may be brought on by diet, age, or habits; it may be local or systemic; and in some cases it may be iatrogenic.