Objectives: Acute pain control after surgery is a crucial fact in health management. Memantine is a N-Methyl-D-aspartate (NMDA) antagonist which is a leading drug for Alzheimer's disease treatment. Design: In this double blind randomized clinical trial, 60 patients candidate for dacryocystorhinostomy are assigned. Choosing envelope number 1 or 2, patients are randomly allocated to memantine or placebo group. Patients, anesthesiologist and the statistician are blinded to the assignments. This clinical trial is in phase 2 / phase 3. Setting and conduct: General anesthesia will be carried out with fentanyl 2 micro gram per kilogram, midazolam 0.12 milligram per kilogram, sodium thiopental 5 milligram per kilogram and atracurium 0.5 milligram per kilogram. After tracheal intubation anesthesia will be maintained with propofol 100 micro grams per kilogram per minute Major inclusion and exclusion criteria: Participants with written consent form and without any serious cardiovascular, respiratory, renal, liver, cerebral disease and addiction are included in the study. Patients whom do not meet all the above criteria are excluded. Intervention: Oral memantin 40 mg (exir darou, Iran) or placebo (flour tablet) will be administered to the patients in each group at entrance to the operation room and before anesthesia. Main outcome measurements: Visual analogue scale and ramsey scale will be used immediately, 1, 2 and 6 hours after surgery, for the measurement of pain and sedation score, respectively. Data will be recorded and compared between groups.