Background <p>Sodium hypochlorite (NaOCl), the primary endodontic disinfectant, is available in both domestic-grade products and specialized medical-grade formulations. The stability and concentration of domestic-grade products are inconsistent, potentially affecting safety and clinical outcomes. This trial compared the impact of domestic-grade versus medical-grade NaOCl on postoperative pain following endodontic treatment in patients with symptomatic irreversible pulpitis.</p> Methodology <p>Sixty participants with symptomatic irreversible pulpitis in mandibular molars were randomly distributed into two groups (<i>n</i> = 30). Following instrumentation with Edge X7 rotary files, Group A received domestic-grade NaOCl irrigation, while Group B was treated with a medical-grade formulation. Participants recorded pain intensity on an 11-point numerical rating scale (NRS) at 6, 12, 24, 48, and 72&#xa0;h.</p> Results <p>Domestic-grade NaOCl was associated with significantly higher pain levels at 6&#xa0;h (median 3 vs. 0; <i>p</i> &lt; 0.05) and 12&#xa0;h (median 2 vs. 0; <i>p</i> &lt; 0.05) compared to medical-grade NaOCl. No significant differences were observed at 24, 48, or 72&#xa0;h.</p> Conclusions <p>Domestic bleach caused significantly higher early postoperative pain than medical-grade sodium hypochlorite due to its unstandardized, higher concentration. Standardized medical-grade irrigants are recommended to maximize patient comfort and clinical predictability.</p> Trial Registration <p>The protocol for this prospective, parallel group, double blinded, randomized clinical trial was registered at ClinicalTrials.gov (<a href="https://register.clinicaltrials.gov/">https://register.clinicaltrials.gov/</a>) on 18/11/2025, registration number (NCT07248189). The trial was retrospectively registered.</p>