76 patients who are candidates for surgery in hospitals affiliated with Islamic Azad University of Tehran are randomly divided into two groups. One hour before surgery, Mirtazapine will be given to the intervention group and placebo to the control group. After transferring the patients to the ward, the pain level of the patients is measured serially. We want to evaluate the preoperative effect of Mirtazapine administration on reducing postoperative pain (pain severity and duration of pain relief).

In this study, we are going to evaluate the preoperative effect of Mirtazapine administration on reducing postoperative pain, in hospitals affiliated with Islamic Azad University of Tehran.

A controlled, single-arm, single-blind, phase 3 clinical trial on 76 patients. Convenience sampling method was used for sampling.

During this single-blind clinical trial study, 76 patients are selected from the patients who are candidates for surgery in the hospitals of Islamic Azad University of Tehran who met the criteria for entering the study, based on the convenience sampling method. After obtaining informed consent, the participants are blinded to study groups. Patients are randomly divided into two groups receiving Mirtazapine and the control group (placebo B-Complex). One hour before the surgery, the first group will be given a 30mg Mirtazapine tablet orally, and the second group will be given a placebo tablet by student under the supervision of an anesthesiologist.

Inclusion criteria: Patients who are candidates for surgery Insensitivity to Mirtazapine Age between 25 and 50 years BMI between 20 and 25 ASA one or two Patient consent to conduct a clinical trial Criteria for not entering the study: Surgeries greater than an hour Emergency surgeries Pregnant women or the possibility of pregnancy Diabetic, cardiac, pulmonary and neurological patients Drug addiction

Intervention group: Mirtazapine Control group: B-complex

Pain severity is measured serially 2, 6 and 12 hours after leaving the operating room.

The intervention group receives mirtazapine with the aim of reducing pain after surgery, while the control group with the same goal is prescribed placebo under the name of B-complex. In this one-sided blind study, the researcher is aware of the recipients of mirtazapine and placebo, but the participants have been blinded to the study groups after obtaining informed consent. The members of the intervention and control groups were randomly selected and there are no special conditions for the classification of patients in this study. The purpose of blinding this clinical trial to the patients receiving the drugs is to investigate the analgesic effects of mirtazapine as best and accurately as possible and to avoid the involvement of confounding factors that affect the results of this study.