The combination of metronidazole (MTZ) and amoxicillin (AMX) has been shown to be a promising adjunctive therapy in the treatment of chronic and aggressive periodontitis. However, the ideal time to use these drugs in periodontics has not been tested. Some research groups suggest the use of systemic antibiotics at the same time the scaling and root planing (SRP) is conducted on the other hand there are groups that suggest its use after the anti-infective therapy. This information has not been studied in the literature, and these definitions are very important, because the use of MTZ and AMX has proved promising in the treatment of periodontitis and treatment protocols need to be better defined. Therefore, the aim of this study is to compare the clinical and microbiological effects of different times of administration of MTZ (400 mg) and AMX (500 mg) in the treatment of chronic and aggressive periodontitis. Sixty-four subjects, 32 with chronic periodontitis (PCrG) and 32 subjects with generalized aggressive periodontitis (GAgP), will be randomly assigned into two treatment groups (n = 32 subjects / group, 16 PCrG and 16 GAgP): Group Test 1; SRP + MTZ + AMX - administered in the begingin of SRP, and Test Group 2: SRP + MTZ + AMX - administered immediately after SRP. Individuals in both groups receive SRP in 14 days, and the individuals in the test will receive a placebo drug for 14 days after the SRP and individuals in the Test group 2 will receive placebo medication during the first two weeks of SRP . Both systemic antibiotics, and their respective placebos, will be given 3x daily for 14 days. All subjects will be evaluated clinically and microbiologically at baseline, and at 3 and 6 months post-SRP. Six sites per tooth will be evaluated for the clinical parameters of probing depth, clinical attachment level and percentage of sites presenting visible plaque, gingival bleeding, bleeding on probing and suppuration. Nine subgingival biofilm samples will be collected and analyzed for 40 individual bacterial species by Checkerboard DNA-DNA Hybridization. Differences in clinical and microbiological parameters between experimental times in each group will be evaluated using the Friedman test and Dunn. The Mann-Whitney test is used to examine the differences between the two treatment groups at different time points. Statistical significance will be set at 5% (p d 0.05).
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