This randomized controlled clinical trial investigates the Effect of Intracanal Cryotherapy Versus Laser Application on Post-Operative Pain and Bacterial Load Reduction in Single-Visit Root Canal Treatment in Patients with Symptomatic Irreversible Pulpitis in Mandibular First Molars. Sixty patients aged 18-40 years with restorable mandibular first molars diagnosed with symptomatic irreversible pulpitis will be enrolled at the Endodontic Department, Faculty of Oral and Dental Medicine, Future University in Egypt. Participants will be randomly allocated into four groups: Group 1 (control) will receive final irrigation with 2.6% sodium hypochlorite (NaOCl) at room temperature (24°C), Group 2 will receive cryotreated NaOCl (2-4°C), Group 3 will receive preheated NaOCl (60°C), and Group 4 will receive laser-activated NaOCl, each applied for one minute. The study will be double-blind for both the operator and the patient. Post-operative pain will be assessed using a Numerical Rating Scale (0-10) at 6, 12, 24, 48, and 72 hours post-treatment. Bacterial load reduction will be measured microbiologically at three stages: after access cavity preparation, after instrumentation, and after final irrigation. Statistical analysis will be performed using SPSS version 24 and Minitab version 16, with a significance level of p \< 0.05. The study has received ethical approval from the Future University Ethics Committee and is conducted without external funding.