• Background: Peridontitis is a group of diseases with complex bacterial etiology that affects a large proportion of the Chilean and worldwide population. Its treatment, based on an anti-infective mechanical therapy including scaling and root planning procedures, has been highly effective in numerous short and long term clinical trials. The use of adjunctive systemic antibiotics has shown significant improvements in the initial therapy results of specific periodontal disease types or patient profiles.

The synergic combination of Metronidazole plus amoxicillin has been widely and successfully used for the treatment of aggressive and severe chronic periodontitis, demonstrating additional benefits in clinical and microbiological parameters over scaling and root planning alone. However, and despite its proven efficacy, there is a lack of evidence that evaluates the impact of antimicrobials intake at different times of the mechanical treatment, emphasizing the necessity of a study that compares, simultaneously, the variation of the adjunctive antibiotic effects when they are administered in different moments through the periodontal therapy course.

Inclusion Criteria:

• a) age of 35 years old or more, b) at least 3 molars in the mouth, c) at least one site in three of the four quadrants with clinical attachment loss ≥ 3mm; d) at least 4 sites in different teeth with probing depth and clinical attachment level ≥ 6mm and bleeding on probing and at least 14 tooth present in the mouth

Exclusion Criteria:

• a) previous professional root planning; b) systemic/topic antibiotic or antiinflammatory treatment, at least 6 months prior to the initiation of the study; c) immunosuppression or need for antibiotic coverage for routine dental therapy; d) systemic condition that could affect the progression of periodontal disease (diabetes); e) pregnancy or lactation; e) allergy or intolerance to metronidazole or amoxicillin; f) smokers