Background: when patients with chronic back pain seek general surgery, it's typical to see them abusing opioids and other non-steroidal anti-inflammatory drugs, Paracetamol, muscle relaxant drugs, and gabapentin. There have been reports of substance abuse and misuse among these patients as a result of the excruciating pain. Patients who have abused opioids and other previous medications over an extended period of time have begun to experience painful postoperative adverse effects. Planning for appropriate preoperative optimization and perioperative management is aided by toxicology screening tests (TST). Aim of the study to determine whether TST by immunoassay followed by gas chromatography is feasible and whether it could deliver information that is more trustworthy than self-reporting. Subjects and Methods: we created a cross section research to track the accuracy of these patients' self-reports and TST tested all abused substances. Results: The data suggests that the relationship between pre-operative drug use confirmed by toxicology screening rather than self-report and intra-operative use of propofol as well as post-operative pain as outcome parameter. There was a statistical significant strong positive correlation between number of drugs detected in TST and induction dose of propofol (Spearman correlation r=0.78), suggesting that the dose of propofol should be raised when the number of drugs used was increased, pain score in the last week (r=0.84), as well as length of hospital stay (r=0.71) with p value <0.001. Conclusion: TST is a valuable test to identify substances taken by patients when they are scheduled for elective surgery because self-report is frequently unreliable.