In this double-blinded randomized clinical trial, patients presenting periodontal intrabony defects requiring regenerative surgery and showing BoP will be included. Experimental sites at T-0 will be randomly treated with gentle debridement and administration of local DOX (test group) or with debridement alone (control group). After 2 weeks (T-1), defects will be treated by means of minimally invasive surgical technique. 14 days after the surgery (T-2) the EHI will be evaluated by a blinded examiner. Patients will be recalled each 2 months up to 12 months (T-3). At T-0, T-1 and T-3 periodontal examination and radiographic analysis will be done.

Inclusion Criteria:

• 1) diagnosis of Stage III or IV periodontitis (Tonetti et al. 2018);
• 2) FMPS and FMBS < 15% at the time of enrollment;
• 3) etiological periodontal therapy completed at least 3 months prior to screening;
• 4) presence of one natural tooth having a vertical defect with residual PPD ≥ 6 mm and a radiographic intrabony component ≥ 3 mm and BoP + requiring periodontal regenerative surgery.

Exclusion Criteria:

• 1) age < 18 years;
• 2) smoking habits (> 10 cigarettes/day);
• 3) contraindications for periodontal surgery;
• 4) systemic diseases affecting periodontal healing;
• 5) pregnancy and lactation;
• 6) history of periodontal surgery at the experimental teeth;
• 7) allergies to doxycycline and tetracyclines;
• 8) assumption of antibiotics in the last 3 months.