This single-centre, randomized, parallel-group clinical trial compared two commonly used final irrigants in single-visit root canal treatment: octenidine dihydrochloride (OCT) and chlorhexidine (CHX). Adults with single-rooted, single-canal teeth diagnosed with asymptomatic apical periodontitis were treated in one visit under local anaesthesia and rubber-dam isolation. After shaping with sodium hypochlorite and smear-layer removal with EDTA, the assigned final irrigant was delivered and sonically activated. In both groups, OCT 0.1 percent or CHX 2 percent was activated using three 20-second cycles, and the solution was refreshed with 2 mL between cycles (approximate total 6 mL). Postoperative pain was recorded by participants on an 11-point Numeric Rating Scale (0 = no pain, 10 = worst pain) at 6, 12, 24, and 48 hours. The primary outcome was pain at 48 hours. Secondary outcomes were pain at earlier time points, use of rescue analgesics within 0-48 hours, and unplanned urgent care within 48 hours. The aim was to determine whether OCT reduces early postoperative pain compared with CHX when used as the final irrigant in single-visit endodontics.

Design and setting. Prospective, randomized, parallel-group, blinded clinical trial conducted at a university dental hospital (Istanbul, Turkey). All treatments were completed in a single visit by an experienced endodontist using local anaesthesia and rubber-dam isolation.

Participants. Adults (18 years or older) with single-rooted, single-canal teeth diagnosed with asymptomatic apical periodontitis on periapical radiographs were eligible. Exclusion criteria included symptomatic apical periodontitis or acute abscess, previous root-canal treatment of the study tooth, systemic conditions interfering with pain assessment or requiring antibiotic prophylaxis, pregnancy or lactation, known allergy to study materials, and use of analgesics or anti-inflammatory drugs within 12 hours before treatment. One patient contributed one tooth.

Randomization, allocation concealment, and masking. Participants were randomized 1:1 to OCT or CHX using computer-generated permuted blocks with stratification by baseline pain (NRS <= 3 vs > 3). Allocation was concealed with sequentially numbered, opaque, sealed envelopes. Irrigants were prepared by a third party in identical opaque syringes labelled A/B. Participants, the care provider (operator), and the outcomes assessor were blinded until database lock.

Interventions. Working length was established with an electronic apex locator and confirmed radiographically when needed. Root canals were prepared with nickel-titanium rotary instruments to size 40/0.04. During shaping, 5.25 percent sodium hypochlorite was used (approximately 2 mL after each instrument change). The smear layer was removed with 17 percent EDTA (about 10 mL) activated with a sonic device, followed by saline. For the final irrigation, the allocated solution was delivered and sonically activated using the same protocol in both groups: OCT 0.1 percent or CHX 2 percent, activated in three 20-second cycles with the solution refreshed with 2 mL between cycles (approximate total 6 mL). A side-vented needle was placed short of the working length. Root-canal obturation was completed in the same session using gutta-percha and an epoxy resin-based sealer with cold lateral compaction. Access cavities were restored with a resin composite. Rescue analgesics were permitted as needed and recorded by patients.

Outcomes. Primary outcome: patient-reported pain on the 0-10 NRS at 48 hours after treatment. Secondary outcomes: NRS pain at 6, 12, and 24 hours; total rescue analgesic consumption from 0 to 48 hours; and unplanned urgent care or adverse events within 48 hours.

Sample size and analysis plan. The a priori sample-size calculation assumed a between-group difference of 1.0 NRS unit (SD 2.0), alpha 0.05, and power 0.80, yielding 100 participants; allowing for attrition, the target was 120. Analyses compared groups at 48 hours and evaluated pain trajectories over time using mixed-effects methods with fixed effects for group and time and a random intercept for participant. Two-sided p < 0.05 was considered statistically significant.

Oversight. The protocol was approved by the Bezmialem Vakif University Clinical Research Ethics Committee (approval E.99606). The trial was conducted in accordance with the Declaration of Helsinki.

Inclusion Criteria:

• Adults aged 18 to 65 years (inclusive)
• Single-rooted, single-canal tooth
• Diagnosis of asymptomatic apical periodontitis on periapical radiographs
• Able and willing to provide written informed consent
• Able to complete a 48-hour pain diary (6 h, 12 h, 24 h, 48 h)

Exclusion Criteria:

• Symptomatic apical periodontitis or acute apical abscess
• Previous root canal treatment of the study tooth
• Systemic condition requiring antibiotic prophylaxis or that interferes with pain assessment
• Pregnancy or lactation
• Known allergy or contraindication to study irrigants/materials
• Use of analgesics or anti-inflammatory drugs within 12 hours before treatment
• Inability to comply with study procedures
• More than one eligible tooth (one patient = one tooth rule; a pre-specified index tooth is selected)

Irrigants prepared by a third party in identical opaque A/B syringes; allocation concealed with SNOSE; participants, care provider (operator), and outcomes assessor were blinded; A/B code broken only after database lock.