Obsessive-compulsive disorder (OCD) is the fourth most common mental disorder in Bangladesh and around the world. Selective serotonin reuptake inhibitors (SSRIs) are commonly administered drugs for treating children and adolescents with OCD. The treatment of this condition has remained unsatisfactory, only 40-60% of patients with OCD respond to SSRIs, and about 40 to 60% of patients with OCD do not completely respond to this drug. Researchers throughout the world are involved to obtain new pharmacotherapy for the treatment of OCD. A good number of evidences have proved that glutamate plays a significant role in the pathophysiology of OCD.N-acetylcysteine (NAC) is an amino acid derivative, is a health supplement, and has neuroprotective effects that target the glutamatergic system. Some recent trials attempted to assess the effect of n-acetylcysteine as an add-on therapy with SSRIs to improve symptoms in OCD patients. Aim: This proposed study is therefore an effort whether there is any role of n-acetylcysteine adjuncts with SSRI produce better improvement than SSRI alone in OCD patients. Method: This study will be a randomized, double-blind, placebo-controlled multicenter trial that will be conducted in the department of pharmacology, BSMMU in collaboration with the department of psychiatry, BSMMU, and Combined Military Hospital (CMH), Dhaka from the day of approval by the IRB to January 2022. A total of 56 patients suffering from OCD will be selected according to inclusion and exclusion criteria. The diagnosis of patients suffering from OCD and the selection of drug and dosage would be performed by a senior professor of the psychiatry department. After completing the necessary formalities including informed consent of the patients, the patient would undergo a selected questionnaire Yale-Brown Obsessive Scale (Y-BOCS-10) to assess the severity of the disease. The patients would be randomly allocated into two arms: control and intervention. Patients in the intervention arm would consist of 30 patients who will receive SSRIs plus NAC. NAC was titrated from 600mg/day and doubling weekly to reach a maximum dose of 2400mg/day (at week-3) for the remaining 7 weeks. On the other hand, the control arm would consist of 30 patients who will receive SSRIs plus a placebo for 10 weeks. The severity of symptoms will be assessed after 4 weeks and 10 weeks follow-up. The brain glutamate level, ALT, AST, serum creatinine, and ECG will be measured at baseline and after 10 weeks. The regularity of medicine intake will be ensured over the telephone, pill count, and from the patient's compliance sheet. Statistical analysis will be obtained by SPSS (statistical package for social science) version 24. Paire't' test will be done to compare between two means of before and after the intervention. Proportion test 'Z' will be done to compare the proportion between the intervention group and the placebo group. The calculated 'p' value may suggest the level of significance (significant at p\<0.05). Ethical consideration: After approval from IRB every eligible patient will be informed about the intervention and the study objectives. Patients will also be informed that they can participate or to withdraw at any time without compromising their medical care. Patient confidentiality will be strictly maintained. Patient's personal data regarding the name, age, sex, and other information will not be disclosed anywhere and will be used only for research purposes.