Aim: The purpose of this study was to compare the anaesthetic efficacy of inferior alveolar nerve block (IANB) using 1.7 ml of 4% articaine with 1:100,000 epinephrine in reversibly and irreversibly inflamed mandibular molars. Materials and methods: Thirty-six adult patients diagnosed with reversible or irreversible pulpitis of a mandibular molar received 1.7 ml of 4% articaine with 1:100,000 epinephrine, using a conventional IA NB. Following administration, teeth were tested with Endo-Ice® (1,1,1,2 tetrafluoroethane; Hygenic) at 5- and 10-minute intervals. The parameters to evaluate were onset of lip numbness, onset of pulpal anaesthesia and a painful or non-painful response during the procedure. Success was defined as none or mild pain upon cavity preparation or during access and instrumentation of the root canals. In case of failure, supplemental anaesthesia was administered. Statistical analysis was performed using Fisher's exact test, data on time to lip anaesthesia were analysed through survival analysis techniques, and response to cold and pain during the procedure by two-way tables. Results: The pain was too severe to proceed in 5.3% of the reversible pulpitis group and 66.7% of the irreversible pulpitis group. The trend for more pain in the irreversible pulpitis group was statistically significant (P 0.001). Pain during the procedure did not show any statistically significant association with response to cold at 5 minutes after IANB (P > 0.999) or time to onset of lip anaesthesia (P = 0.122). Conclusion: The IANB using 1.7 ml of 4% articaine with 1:100,000 epinephrine is much more likely to be inefficient in patients presenting with irreversible pulpitis than in patients with reversible pulpitis. In addition, the absence of pulpal response to Endo-Ice does not guarantee pain-free root canal procedures. Keywords: endodontic anaesthesia, inferior alveolar nerve block (IANB), irreversible pulpitis