Vitamin D deficiency (VDD) is a prevalent global health concern that has been linked to altered pain perception through its effects on inflammatory pathways, nociceptor activity, and neurotrophic signaling. While its role in chronic pain has been widely studied, limited evidence exists regarding its impact on acute postoperative dental pain. This triple-blinded randomized controlled clinical trial aims to evaluate the effect of moderate-dose vitamin D supplementation on postoperative pain following single-visit root canal treatment (RCT) in patients with symptomatic irreversible pulpitis affecting mandibular first molars. A total of 54 vitamin D-deficient patients (serum 25(OH)D \<30 nmol/L), aged 20-45 years, will be randomly allocated into three groups: a single preoperative dose group (150,000 IU vitamin D₃), a fractionated dose group (50,000 IU over three consecutive days), and a placebo group. All participants will undergo standardized single-visit RCT procedures. Postoperative pain will be assessed using a Modified Visual Analog Scale (VAS) at baseline and at 6, 12, 24, and 48 hours post-treatment, while analgesic intake will be recorded as a secondary outcome. It is hypothesized that vitamin D supplementation will significantly reduce postoperative pain levels and analgesic consumption compared to placebo. This study seeks to explore vitamin D as a safe, cost-effective adjunct in endodontic pain management, with potential implications for improving clinical outcomes in patients with VDD undergoing RCT.