It has been shown that iatrogenic preparation damage increases caries progression and the need for restorative therapy of the adjacent tooth. The aim of this study was to quantify the damage of the adjacent tooth using three different preparation techniques: (1) diamond bur alone; (2) diamond bur combined with the EVA system and the Cavishape file; (3) diamond bur combined with an axial margin trimmer. All 136 preparations were performed by dental practitioners in their own practice. Examination of the area directly opposite to the axial box margin revealed significantly less damage (p < 0.01) using additionally the EVA system with the Cavishape file or the axial margin trimmer compared to using only diamond burs. The area opposite between the two box margins, which was in all three methods prepared only by the diamond bur, showed no significant differences between the methods under study. There were only a few teeth (overall 5%) with no damage of the adjacent surface. In order to reduce iatrogenic damage for the adjacent teeth the preparation technique has to be modified.