this study aims to assess the effect of photobiomodulation using low-level diode laser therapy on postoperative pain after single-visit root canal treatment of mandibular molars with symptomatic irreversible pulpitis and apical periodontitis.

Inclusion Criteria:

• . Mature multirooted mandibular molar teeth with: i. Pre-operative sharp pain marked on the VAS scale by reading not less than 6. ii. Clinical diagnosis of symptomatic irreversible pulpitis with periapical periodontitis.

iii. Normal periapical radiographic appearance or slight widening in lamina dura. iv. Positive response and pain provoked by percussion.

- Patients who agree to provide written consent and attend for recall appointments.

Exclusion Criteria:

• Patients who used NSAIDs in the 24 hours before treatment.

• Medically compromised patients having significant systemic disorders. (ASA III or IV).

• Patients with two or more adjacent teeth requiring endodontic treatment.

• Pregnant or nursing females. 5. Teeth that have:

  i. Periodontal affection (with pocket depth greater than 5mm, associated with swelling orfistulous tract, or greater than grade I mobility).

ii. No possible restorability. iii. Signs of pulpal necrosis; associated sinus tract or swelling.