The goal of this clinical trial is to compare Opioid Free and Opioid Based Anaesthesia in patients undergoing sleeve gastrectomy. The main questions it aims to answer are: * Will the total dose of intraoperative opioid be reduced? * Will there be difference in pain scores between groups? Participants will be managed with the Nociceptive Level Index algorithm to guide intraoperative analgesia. The Opioid Free Anaesthesia Group will be administered the Multimix infusion (Magnesium sulfate, dexmedetomidine, ketamine). The Opioid Based Anaesthesia Group will receive fentanyl as a bolus dose and remifentanil infusion. Rescue fentanyl bolused will be injected as appropriate according to nociceptive level (NOL) values.