AbstractThis randomized, parallel, controlled trial assessed the effect of fluoride varnish, ozone and octenidine on white spot lesions (WSLs) and caries during orthodontic treatment. Patients were enrolled between 1st September 2017 and 31st August 2020 at initiation of orthodontic treatment in Department of Interdisciplinary Dentistry Pomeranian Medical University in Szczecin, Poland. All participants were randomly assigned to four study and one control groups using number random generator. However, investigators were not blinded due to the nature of the study. Groups I, II, III, IV had professional cleaning and varnishing (5% NaF) every 4 weeks. Groups II and IV had in-office ozone therapy before varnishing, groups III and IV received domestic octenidine mouthrinse. Group K had no professional hygienic or prophylactic procedures. WSLs were assessed at T0 and then every 4 weeks (T1–T4) and caries—at T0 and T4. The specific objective was to assess the influence of fluoride varnish, ozone and octenidine on the incidence of white spot lesions and caries during orthodontic treatment. The primary outcome of this report was the highest number of WSLs in group K and the lowest percentage of patients with WSLs in group IV. Each group comprised 30 randomized participants; they were all analyzed. No WSLs were found at T0, but they were stated in all groups at T4. The numbers of patients with WSLs significantly increased between T0-T4 in groups I and K. Group IV had the lowest percentage of patients with WSLs in T1-T4. WSLs in group IV were found no earlier than at T2. Group K had the highest percentage of WSLs at T4: 26%. At T0 all the groups had DMFs above 0 with a significant increase at T4. No side effects of the introduced prophylaxis were observed in any group. Caries is an important problem of fixed orthodontic treatment. Even an extremely intensive prophylaxis could not completely prevent WSLs and caries. Simultaneous application of fluoride varnish, ozone gas exposure and octenidine appears to have a beneficial effect in limiting the development of WSLs.Trial registration: NCT04992481.