Objectives To evaluate the effects of 10% carbamide peroxide (CP) with two different thickeners, carbopol (CPc) and natrosol (CPn), on color variation (CV), tooth sensitivity (TS), and cytotoxicity (CC).MethodsSeventy subjects were distributed into the CPc or CPn groups (n=35), in a parallel group, randomized, controlled, single-blind clinical trial. Bleaching gels were used by volunteers for 4h daily for 2weeks. Color evaluation was performed using a reflectance spectrophotometer, before bleaching treatment (BT), immediately after the first and second weeks of BT, and 1week and 1month after BT ended. TS was evaluated using two pain scales, before, during, and after BT. CC was evaluated using MTT after exposure of MDPC-23 cells to the bleaching gels for 4h. Epoxy replicas of the subjects teeth were made before and after BT and analyzed using a scanning electronic microscope. The data was analyzed using statistical methods.ResultsCV and TS showed similar variation between both bleaching gels (p0.05). None of the protocols affected cellular metabolism or the surface morphology of enamel.ConclusionsBleaching gels with carbopol and natrosol as thickening agents produced similarly effective tooth bleaching and TS, but did not cause cytotoxicity.Clinical relevanceNatrosol could be an alternative as a thickener used in bleaching gels due to its similar bleaching effect and TS when compared with Carbopol Ver menos