Background <p>This study aimed to investigate whether adjunctive photodynamic therapy (PDT) could reduce postoperative pain following root canal treatment of mandibular posterior teeth performed by undergraduate dental students.</p> Methods <p>Patients treated in the undergraduate clinic were included if they required endodontic treatment of mandibular posterior teeth and had no systemic contraindications or acute infections. The participants were divided into two groups: conventional final irrigation without activation (CG) and adjunctive PDT application (PDT group). Both groups received 2.5% NaOCl and 17% EDTA during the second visit; in the PDT group, methylene blue was applied and activated with PDT. Postoperative pain was assessed using a numeric rating scale (NRS) at 24, 72&#xa0;h, and 7 days. Analgesic consumption and the occurrence of flare-ups requiring unscheduled visits were also recorded. Statistical analyses were performed with a significance level of 0.05.</p> Results <p>A total of 92 patients were analysed (47 in the PDT group and 45 in the CG). Pain levels decreased significantly over time in both groups (<i>P</i> &lt; 0.05). The PDT group demonstrated significantly lower pain scores at 72&#xa0;h compared with the control group (<i>P</i> &lt; 0.05). Medication use was lower in the PDT group: three patients required analgesics, and none needed antibiotics, compared with six patients (nine tablets) using analgesics and three receiving antibiotics in the CG.</p> Conclusion <p>Within the limitations of this retrospective comparative cohort study, adjunctive photodynamic therapy may be a useful adjunct to endodontic treatment in mandibular posterior teeth treated by undergraduate students.</p> Trial Registration <p>This study was registered with <a href="http://www.clinicaltrials.gov">www.clinicaltrials.gov</a> (ClinicalTrials.gov Identifier: NCT07254260) on 14 November 2025 (retrospectively registered).</p>