Randomized clinical trials and systematic reviews have shown that the use of the combination of metronidazole (MTZ) and amoxicillin (AMX) significantly improves the benefits of the SRP in the treatment of subjects with periodontitis. However, the optimal dosage and interval of administration of these antibiotics are still unclear. Therefore, the aim of this study was to compare the clinical and microbiological outcomes of different dosages and duration of the systemic administration of the combination of MTZ and AMX as adjunct to scaling and root planing (SRP) in the treatment of generalized chronic periodontitis (GChP). One hundred ten subjects will be randomly assigned to receive SRP plus placebo, or SRP combined with 250 mg or 400 mg of MTZ, plus AMX (500 mg) TID, for either 7 or 14 days. Subjects will be clinically and microbiologically monitored up to 1 year post-therapy.