The aim of this randomized clinical trial is to clinically evaluate the effectiveness of the adjunctive use of photodynamic therapy (PDT) with an light-emitting diode (LED) light source in the treatment of chronic periodontitis.

Inclusion Criteria:

1. clinically diagnosis of advanced chronic periodontitis with more than 3 mm clinical attachment loss
2. presence of at least two teeth with probing depth of 4 to 6 mm without furcation involvement in each quadrant
3. at least 16 remaining teeth
4. no periodontal treatment in the past 12 months
5. no evidence of systemic disease related to periodontal problems
6. no use of antibiotics for the past 6 months
7. No smoking more than 10 cigarettes per day
8. no addiction to any kind of drugs
9. no pregnancy