Aim: The aim was to assess the success of inferior alveolar nerve block when adding magnesium sulphate to mepivacaine HCl in comparison to mepivacaine HCl alone in patients with symptomatic irreversible pulpitis in mandibular molars. Methodology: Patients diagnosed with symptomatic irreversible pulpitis were assigned randomly into 2 groups (n=24). Initial pain data were diagnosed with cold spray and collected on a numerical rating scale chart. Both groups received 1.8 ml of local anesthetic solution as a standard inferior alveolar nerve block. In the control group, the local anesthetic solution contained 1.8% mepivacaine HCl while in the experimental group contained 1% magnesium sulphate and 1.8% mepivacaine HCl. Pain was assessed 15 minutes after injection using cold spray, during access and canal negotiation. Success of local anesthesia was defined as recording no or mild pain. Results: The success of inferior alveolar nerve block was 70.8% for the magnesium sulphate group and 50% for the control group. There was no significant difference between the 2 groups (P > 0.05). Conclusion: The combination of mepivacaine– magnesium sulphate achieved a higher success rate for inferior alveolar nerve block than that of mepivacaine alone, but there was no significant difference between both groups.