Background: The anaesthetic sector makes substantial use of local anaesthetic drugs, although anaesthetists have challenges due to the diverse local anaesthetics' short acting times. Numerous perineural adjuvants have been explored to speed up the onset and prolong the analgesic effects of nerve blocks. The role of central sensitizations and N-methyl-D-aspartate (NMDA) receptors in post-operative pains has been highlighted by the acquaintance with pain mechanisms. Objective: The current study aimed to compare the effect of magnesium sulfate (100 mg in 1mL volume) and Dexmedetomidine (100 mcg in 1mL volume) added to lidocaine (20 ml 2%) on the onset time and duration of supraclavicular brachial plexuses block in patients undergoing upper limb surgery. Patients and methods: This prospective randomized-controlled study included 60 patients that were subjected to surgical procedure in upper limb as part of the standard anesthetic techniques. Results: There was statistically significant increase of the duration of motor and sensory block among group D than in group M. Duration of motor block for group M was 277.17 ± 54.34 min versus 482.50 ± 72.75 min for group D. Time to first analgesia for group M was 5.10 ± 1.56 h versus 8.70 ± 3.38 h for group D. A statistically high significant increase total dose of analgesia consumed was detected for group M as compared to group D (8.80 ± 2.44 mg for group D versus 12.53 ± 4.03 mg for group M). Conclusion: Addition of magnesium sulfate or dexmedetomidine to lidocaine in supraclavicular brachial plexus block prolong the duration of sensory and motor block. Both magnesium sulfate and Dexmedetomidine groups showed improved postoperative analgesia with less analgesic requirements.