The goal of this study is to determine if patients with chronic periodontitis when undergoing with scaling and root planing (SRP) plus photodynamic therapy in comparison with SRP alone result in improved clinical, microbiological and biochemical outcomes.

This research is orientated to find an effective way to kill oral microorganisms that cause periodontitis and gingivitis with no problems of resistance, as it happens when we use antibiotics. The efficacy of this low power laser has been demonstrated in numerous fields in medicine and have been use it for years. The application of the photodynamic therapy is easy since we only have to fill the periodontal pocket with the photosensitive agent and apply the low power laser in that area. The photosensitive agent stain the bacteria immediately and the laser disrupts its membrane by a free-radical reaction. Improving the health status of the patients in a few weeks after treatment.

Inclusion Criteria:

1. Chronic generalized moderate-advanced periodontitis: based on "Classification System for Periodontal Diseases and Conditions" Armitage 1999. (24).Extension of the 30 % of affected zones. Severity base on clinical attachment loss: moderate = 3-4 mm of CAL, severe= > or equal 5 mm CAL.
2. The subject is diagnosed with moderate or advanced chronic periodontitis.
3. The subject has at least 4 sites with PD of ≥5 mm + bleeding in at least two quadrants of the mouth.
4. A signed Informed Consent will be obtained.
5. The subject is an adult male or female > 18 years of age.
6. The subject has >18 fully erupted teeth, at least 3 teeth in each quadrant with one or more posterior teeth per quadrant (3º molars excluded).
7. The subject is willing and able to return for treatment and evaluation procedures scheduled throughout the course of this clinical study.

Exclusion Criteria:

1. The subject is pregnant or nursing or plans to become pregnant in the next 6 months.
2. The subject is a current smoker.
3. The subject has an active malignancy of any type by subject report.
4. The subject has chronic disease or diminished mental capacity that would mitigate the ability to comply with the protocol.
5. The subject has any significant chronic disease (either acute or chronic) or who is taking a medication with concomitant oral manifestations that in the opinion of the investigator would interfere with evaluation of safety or efficacy of the study.
6. The subject has an active periapical abscess or periodontal abscess or a history of acute necrotizing ulcerative gingivitis.
7. The subject has been treated with antibiotics within the 3-month period prior to beginning the study or any systemic condition which requires antibiotic coverage for routine periodontal procedures (e.g. heart conditions, joint replacements, etc.) by report of the subject.
8. The subject has a known allergy to Methylene Blue, polimetil metacrilate or chlorhexidine.
9. The subject Has severe glucose-6-phosphate dehydrogenase (G6PD) deficiency by subject report.
10. The subject currently uses anti-coagulant therapy at therapeutic doses.
11. The subject is currently using any photosensitizing medications.
12. Patients who have received SRP or Periodontal supportive therapy in the last 6 months
13. It is considered > 30% of plaque of an index of 2 or more in the Silness y Löe plaque index the patient in the study.