This randomized clinical study aims to evaluate the effect of different sodium hypochlorite concentrations (1%, 2.5%, and 5.25%) used during root canal retreatment on intracanal levels of Substance P, CGRP, and Opiorphin, as well as postoperative pain.

Patients requiring non-surgical root canal retreatment will be randomly assigned to one of three irrigation concentration groups. Intracanal samples will be collected before and after irrigation to measure neuropeptide levels. Postoperative pain will be assessed using a visual analog scale (VAS) at predetermined time intervals.

The results of this study may help clarify the relationship between irrigant concentration, inflammatory mediators, and postoperative pain following root canal retreatment.

This prospective, randomized, single-center clinical study is designed to investigate the effects of different sodium hypochlorite (NaOCl) concentrations (1%, 2.5%, and 5.25%) used during non-surgical root canal retreatment on intracanal levels of Substance P, calcitonin gene-related peptide (CGRP), and Opiorphin, as well as postoperative pain intensity.

Patients diagnosed with teeth requiring root canal retreatment will be included according to predefined inclusion and exclusion criteria. Following access cavity preparation and removal of previous root canal filling material, intracanal samples will be obtained prior to irrigation. Participants will then be randomly allocated to one of three irrigation protocols using different NaOCl concentrations. A second intracanal sample will be collected after irrigation.

Neuropeptide levels will be quantified using enzyme-linked immunosorbent assay (ELISA). Postoperative pain will be recorded using a visual analog scale (VAS) at specified time intervals following treatment.

The primary outcome measures are the changes in intracanal Substance P, CGRP, and Opiorphin levels. The secondary outcome measure is postoperative pain intensity. This study aims to contribute to a better understanding of the biological and clinical effects of irrigant concentration during root canal retreatment.

Inclusion Criteria:

• Patients aged between 18 and 65 years.
• Systemically healthy individuals (ASA I or II)
• Patients requiring non-surgical root canal retreatment of a previously treated permanent tooth
• Patients who provide written informed consent.

Exclusion Criteria:

• Patients with systemic diseases that may affect pain perception or inflammatory response.
• Use of analgesics, anti-inflammatory drugs, or antibiotics within 7 days prior to treatment.
• Pregnant or lactating women.
• Teeth with root fractures, perforations, advanced periodontal disease, or non-restorable teeth.
• Teeth with open apices or immature root development.
• Presence of acute apical abscess with swelling or sinus tract.
• Patients with known allergy to sodium hypochlorite.
• Inability to achieve adequate isolation with rubber dam.
• Patients unable to attend follow-up or complete pain assessment forms.

Individual participant data (IPD) will not be shared due to patient confidentiality and institutional data protection policies. The dataset contains identifiable clinical information collected as part of a single-center academic thesis study.

Participants will be blinded to the sodium hypochlorite concentration used during retreatment. Laboratory personnel performing ELISA analysis and outcome assessors evaluating postoperative pain will also be blinded to group allocation. The clinician performing the procedure will not be blinded.