The purpose of this study is to find difference in Minimum Alveolar Concentration(MAC) for maintaining Bispectral Index (BIS) below 50 between chronic opioid users and opioid naive patients.

1. Patients selection in elective surgery which need general anesthesia.
2. MAC BIS50 data was collected in each patients (by blinded investigator)
3. The data collection will be continued for enough sample size for using Dixon Up Down Method Analysis
4. After Data collection, MAC BIS below 50 will be analyzed in both groups ( Chronic opioid users versus opioid naive patients)

Inclusion Criteria:

• The patients(ASA class I-III) who receive elective surgery
• signed in informed consent to participate in this trial voluntarily.

Exclusion Criteria:

• History of Monoamine Oxidase (MAO) inhibitor use
• Chronic alcoholics
• Acute alcohol abuser
• Contraindication for inhalation agent use (Sevoflurane)
• BMI >30
• History of medication use, which affect in the minimum concentration of sevoflurane;(barbiturates, chlorpromazine, diazepam, hydroxyzine, verapamil, marijuana)
• Craniotomy