AUGMENTATION THERAPY involves the addition of a second drug, such as mood stabilizers, antipsychotics, and nutritional supplements, to a primary antidepressant treatment. Studies on adding FOLIC ACID to a preexisting antidepressive regimen as a form of AUGMENTATION THERAPY have had different and even controversial results. This study aimed to determine the effects that adding FOLIC ACID to a pharmaceutical diet with CITALOPRAM has on the treatment of depression. This double-blind randomized clinical trial was conducted in Kashan, Iran on 90 patients who suffered from depression. Patients were allocated to study groups using random permuted blocks. One group (n= 45) received a dosage of 20 mg CITALOPRAM in combination with 2.5 mgfolic acid on a daily basis, and the other group (n= 45) received the same daily dose of CITALOPRAM with a placebo for eight weeks. To measure the severity of each patient’s depression, the Beck depression inventory II (BDI-II) questionnaire was used prior to starting the antidepressant therapy and was repeated four, six, and eight weeks after beginning the treatment. A reduction from the original BDI-II scores that was greater than 50% was considered to be a response to treatment. The average depression scores before treatment were 30.11±10.41 in the intervention group and 31.24±10.26 in the control group (P= 0.6). At the end of the study, the depression scores in the intervention and the control groups were 13.31±6.57 and 19.11±8.59, respectively (P< 0.001). A reduction in the average depression scores of the intervention group was statistically significant after six and eight weeks (P= 0.01 and P= 0.001, respectively). At the end of the study, the frequency of response to treatment was 73.3% in the intervention group and 40.0% in the control group (P< 0.001). FOLIC ACID, when used as a complementary therapy, can improve a patient’s response to antidepressants used for the treatment of major depression.