The aim of this prospective, randomized, double-blind, clinical trial is to evaluate the effect of a preoperative, single, oral dose of aceclofenac (100 mg) on postendodontic pain at 6, 24 and 48 hours, postoperative pain at injection area at 6, 24 and 48 hours and analgesic intake after single-visit root canal treatment.

• Patients will be clinically and radiographically examined and their eligibility will be assessed and preoperative pain will be measured using Heft-Parker VAS. Eligible patients will be treated in one visit.
• Patients will be randomly assigned to one of 2 groups: experimental group (taking a 100 mg of Aceclofenac) and the control group (taking a placebo tablet). Each tablet will be taken one hour before the initiation of single-visit endodontic therapy. Each participant will receive a standard inferior alveolar nerve block injection of 1.8 ml of 2% mepivacaine with 1:100,000 epinephrine. After endodontic treatment, patients will be given postoperative instructions and informed, in case of pain, to receive ibuprofen 200 mg as rescue medication.
• Postendodontic pain intensity and incidence at the different pain categories (No, mild, moderate, severe) will be assessed 6, 24, and 48 hours postoperatively using Heft-Parker visual analogue scale (HP-VAS). Postoperative pain at injection area will be assessed at 6,24 and 48 hours using HP-VAS. Analgesic intake throughout the 48 hours will be recorded.

Inclusion Criteria:

• Patients in good health (American Society of Anesthesiologists Class I or Class II).
• Patients having symptomatic irreversible pulpitis in one of their mandibular molars
• Age is 18 years or older.
• Patients who can understand Heft- Parker Visual Analogue Scales.
• Patients able to sign informed consent.

Exclusion Criteria:

• Patients allergic to aceclofenac or Mepivacaine.
• Pregnant or nursing women.
• Patients having active pain in more than one molar in the same quadrant.
• Administration of analgesics within 12 h before the administration of the study drug.