Gum disease require a lifetime supportive periodontal care aiming at preventing additional inflammation and bone resorption. During this phase it is also relevant to keep germs under acceptable levels through proper daily hygiene although patient's cooperation tends to decrease over time.

There are conflicting opinions regarding combination of dental scaling with antibiotics. This is why the present study was designed to evaluate the clinical and microbiological effects of combined therapy 1 year following supportive periodontal care. Fifty patients who voluntarily decide to participate will receive dental scaling associated with two different antibiotics or placebo capsules. Paper points will be used to collect dental plaque samples and levels of bacteria will be determined. Dental specialists will also perform clinical examinations. Patients would be available for 4 dental appointments: prior to and 3, 6 and 12 months after dental scaling.

The benefits of amoxicillin (AMX) plus metronidazole (MTZ) adjunctive to scaling and root planing (SRP) require further investigation, particularly in individuals with a high occurrence of recurrent periodontal pockets under periodontal maintenance therapy (PMT). The aim of the study was to evaluate the clinical and microbiological outcomes of systemic AMX+MTZ adjunctive to SRP in patients with recurring sites [probing depth (PD) ≥5 mm) under PMT, after 1 year of follow-up, compared to SRP alone.

Methods: A randomized controlled clinical trial will be conducted with 50 individuals in a PMT program, which will be randomly allocate in two groups: control group (SRP and placebo; n=24) and test group (SRP and systemic MTZ+AMX; n=23). Participants of the present study will be selected from an open cohort study comprising individuals under a PMT program, who were monitored over 12 years of consecutive recall visits (from August 2009 to April 2023). Periodontal clinical parameters and subgingival biofilm samples will be collected by trained and calibrated examiners at baseline (T1), 3 (T2), 6 (T3) and 12 (T4) months after treatment. Microbiological analyses will be performed at T1, T2 and T4 by real-time quantitative polymerase chain reaction.

The percentage of residual pockets will be listed. Additionally, an overall mean PD value will be calculated for the treated sites initially measuring ≥ 5mm and clinical attachment loss (CAL) ≥ 3mm. Statistical testing for normality with respect to the distribution of study outcomes (clinical parameters) will be performed using the Kolmogorov-Smirnov test. Parametric and non-parametric tests (Chi-squared, Wilcoxon and Friedman tests) will be performed when appropriate. Spearman's correlation (r) will be used to evaluate the relationship between bacterial counts and periodontal condition between groups and evaluation times. All analyses will be performed using statistical software Statistical Package for the Social Sciences (SPSS) (p < 0.05).](streamdown:incomplete-link)

Inclusion Criteria:

• individuals under a PMT program
• monitored over ~13 years of consecutive recall visits (from August 2009 to May 2023).
• individuals reclassified as stage III and IV periodontitis
• not having presented the following clinical endpoint in two consecutive PMT visits - presence of ≤4 sites with PD ≥5 mm
• non-diabetics
• non-smokers
• no antibiotic use for any purpose within 3 months prior to entering the study

Exclusion Criteria:

• pregnant or lactating women
• smokers
• presence of systemic diseases that could affect the progression of periodontitis long-term administration of anti-inflammatory drugs,
• need for antibiotic premedication for routine dental therapy and
• allergy to metronidazole or amoxicillin

A random sequence will be generated (Excel, Microsoft Office 365) and participants will be allocated in the groups according to sequential numbered opaque envelopes. Each envelope containing one of the two treatments will be open only at the time of the periodontal procedure by the operator. Participants, as well as the researcher that evaluated treatment clinical outcomes won't know which group they were assigned.

It will be ensure that the patient, the main researcher, the operator, the examiner and the statistician will be unaware of study groups.