Subject Registration and Randomization Subjects that fulfil all the inclusion criteria, do not meet any exclusion criteria, and have signed the informed consent will be registered into the study by the centers Münster or Würzburg (visit 1). At visit 2 study subjects will be randomly assigned to the test group using the carbonate/hydroxyapatite nanocrystal dentifrice or the control group using a zinc-fluoride containing dentifrice. Patient randomization is warranted by using a randomization list (block randomization with 4 subjects per block). Designation of a study subject as a smoker/non-smoker will be based on recording carbonmonoxide concentration in exhaled air (non-smoker: less than 3 ppm CO in exhaled air; smoker: equal to or more than 3 ppm).