Aim: The aim of this prospective, randomized, double blind, controlled study was to evaluate the effect of preoperative oral administration of aceclofenac on the success of mandibular inferior alveolar nerve block (IANB) with 2% Mepivacaine containing 1:100,000 epinephrine in patients with symptomatic irreversible pulpitis.Methods: Seventy endodontic emergency patients diagnosed with irreversible pulpitis of a mandibular posterior tooth randomly received, in a double-blind manner, identical tablets of either 100 mg aceclofenac or placebo 60 minutes before the administration of 1.8 ml of 2% Mepivacaine containing 1:100,000 epinephrine as IANB. The tooth in question was checked every 5 min for 15 min for lip numbness and was tested with cold spray test and the patient was asked to rate his/her pain on Heft Parker visual analogue scale (VAS). Access cavity was performed. In case of pain during the treatment, the patients were asked to rate the pain Heft Parker VAS. The success of the IANB was defined as the ability to access and instrument the tooth with no or mild pain.Results: The success rate for the IANB was 68.6% with aceclofenac and 62.9% with placebo, with no statistically significant difference (p=0.85) between the two groups.Conclusion: Preoperative administration of a single dose of 100 mg aceclofenac did not seem to affect the anesthetic success of IANB using 2% mepivacaine with 1:100,000 epinephrine in patients with mandibular molars suffering from symptomatic irreversible pulpitis.