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The purpose of this study was to determine the anesthetic efficacy of the supplemental intraosseous injection, using the X-tip system in an apical location, in mandibular posterior teeth diagnosed with irreversible pulpitis when the conventional inferior alveolar nerve block failed. Thirty-three emergency patients, diagnosed with irreversible pulpitis of a mandibular posterior tooth, received an inferior alveolar nerve block and had moderate-to-severe pain on endodontic access. The X-tip system was used to administer 1.8 ml of 2% lidocaine with 1:100,000 epinephrine. The X-tip injection site was 3- to 7-mm apical to the mucogingival junction of the affected tooth. Success of the X-tip intraosseous injection was defined as none or mild pain on endodontic access or initial instrumentation. The results of this study demonstrated that 6 of 33 (18%) X-tip injections resulted in backflow of anesthetic solution into the oral cavity; none were successful in obtaining anesthesia. Twenty-seven of the remaining 33 X-tip injections (82%) were successful. We conclude that when the inferior alveolar nerve block fails to provide profound pulpal anesthesia, the X-tip system, when used in an apical location and when there was no backflow of the anesthetic solution into the oral cavity, was successful in achieving pulpal anesthesia in mandibular posterior teeth of patients presenting with irreversible pulpitis.