Aim To evaluate the effect of low-level laser therapy (LLLT) on the success rate of inferior alveolar nerve blocks (IANB) in mandibular molar teeth with symptomatic irreversible pulpitis (SIP). Methodology Eighty-eight patients who were diagnosed with SIP were randomly divided into two groups: the group in which only IANB was applied and the group in which IANB + LLLT was applied. IANB was applied to patients in the control group with 4% articaine. LLLT was applied to the patients in the experimental group in addition to IANB. The pain experienced during the operation was evaluated using a visual analog scale. If the patients reported moderate or severe pain during the treatment, the IANB was defined as unsuccessful. Pearson's chi-square test was used to analyse anaesthetic success rates. Results Whilst the anaesthesia success rate was 34% in the group where only IANB was applied, it was 57% in the group in which LLLT was applied in addition to IANB. There was a significant difference between the groups (p = .032). Conclusions The application of LLLT to support IANB in mandibular molar teeth with SIP increased the success of anaesthesia. However, it was insufficient for a complete pulpal anaesthesia.