The purpose of this prospective, randomized, double-blind, placebo-controlled study was to determine the effect of preoperative oral administration of ibuprofen or dexamethasone on the success rate of inferior alveolar nerve block (IANB) in patients with symptomatic irreversible pulpitis. Seventy-eight patients with irreversible pulpitis were randomly divided into 3 groups (26 per group) and given one of the following at 1 hr prior to performing local anesthesia: a placebo; 400 mg ibuprofen; or 4 mg dexamethasone. Each patient recorded their pain level on a visual analog scale before taking the medication or placebo, at 15 min after completion of IANB, and during treatment if pain occurred. The success of the anesthesia was defined as no or mild pain at any stage during the endodontic procedure. The success rate of the IANB was 38.5, 73.1, and 80.8% with the placebo, ibuprofen, and dexamethasone, respectively. Both ibuprofen and dexamethasone were significantly more effective than the placebo. No significant difference was observed, however, between the two experimental medications in terms of effectiveness. The results of the present study suggest that premedication with ibuprofen or dexamethasone increases the success rate of an IANB in patients with symptomatic irreversible pulpitis in the mandibular molars.