This study aims to evaluate the clinical effectiveness of antimicrobial photodynamic therapy (aPDT) as an adjunct to scaling and root planing (SRP) in the treatment of periodontitis. A split-mouth design will be used, in which one hemiarch receives conventional SRP and the contralateral hemiarch receives SRP combined with aPDT. Clinical periodontal parameters, including probing pocket depth (PPD), clinical attachment level (CAL), bleeding on probing (BOP), plaque index (PI), and gingival index (GI), will be assessed at baseline and 30 days after treatment. The study seeks to determine whether adjunctive aPDT improves periodontal outcomes compared with conventional non-surgical periodontal therapy alone.

Periodontitis is a chronic inflammatory disease characterized by the destruction of the supporting tissues of the teeth and is one of the leading causes of tooth loss worldwide. Scaling and root planing (SRP) is considered the gold standard non-surgical treatment; however, complete elimination of periodontal pathogens may be difficult in deep periodontal pockets and anatomically complex sites.

Antimicrobial photodynamic therapy (aPDT) has emerged as a minimally invasive adjunctive treatment. This technique combines a photosensitizing agent, toluidine blue O, with low-level laser irradiation to produce reactive oxygen species capable of destroying periodontal pathogens while minimizing damage to host tissues.

This study will be conducted at the Dental Clinic of the Universidad Católica de Santiago de Guayaquil, Ecuador. Thirty adult patients diagnosed with periodontitis will be enrolled. A split-mouth randomized design will be used, with one hemiarch receiving SRP alone (control) and the contralateral hemiarch receiving SRP plus aPDT (test group).

Clinical parameters including probing pocket depth (PPD), clinical attachment level (CAL), bleeding on probing (BOP), plaque index (PI), and gingival index (GI) will be recorded at baseline and at 30-day follow-up. The primary objective is to compare the clinical effectiveness of adjunctive aPDT versus conventional SRP alone in improving periodontal health outcomes.

The study has been approved by the Ethics Committee for Research in Human Subjects of the Universidad Católica de Santiago de Guayaquil (CEISH-UCSG) and will be conducted in accordance with ethical principles for human research.

Inclusion Criteria:

• Adults aged 18 to 65 years.
• Diagnosis of Stage I, II, III, or IV periodontitis according to the 2018 World Workshop Classification.
• Presence of at least two hemiarches with periodontal pockets ≥5 mm suitable for a split-mouth design.
• Systemically healthy individuals.
• No periodontal treatment received within the previous 6 months.
• No use of antibiotics, anti-inflammatory drugs, or antiseptic mouth rinses within the previous 3 months.
• Ability and willingness to provide written informed consent and attend follow-up visits.

Exclusion Criteria:

• Uncontrolled systemic diseases that may affect healing or immune response.
• Use of antibiotics, corticosteroids, or immunosuppressive medications within the previous 3 months.
• Active smokers consuming more than 10 cigarettes per day.
• Pregnant or breastfeeding women.
• Presence of dental implants, defective restorations, or teeth with Grade III mobility in the selected study sites.
• Known allergy to toluidine blue O or any component of the photosensitizer.
• Individuals unable or unwilling to attend the 30-day follow-up visit.

A decision regarding the sharing of individual participant data (IPD) has not yet been made. Data sharing plans will be determined after study completion, publication of primary results, and in accordance with institutional policies, ethical approvals, participant confidentiality requirements, and applicable regulations.

Participants will not be informed which hemiarch receives adjunctive antimicrobial photodynamic therapy. Clinical measurements and outcome assessments will be performed by an examiner blinded to treatment allocation whenever possible.