Opalescence Endo, Mini Refill: 2 x 1.2 ml Syringes. Endodontic Whitening System. A specially formulated 35% hydrogen peroxide whitening gel to whiten endodontically treated, discolored teeth using the 'walking bleach' technique. Once Opalescence Endo is delivered to the pulp chamber, it should be sealed with a temporary cement, and left in the tooth for three to five days. 2 - 1.2 ml syringes