Background: Chemotherapy-induced nausea and vomiting (CINV) is an important therapy related side effects in the treatment of malignancies. Breast cancer patients receive a highly-emetogenic chemotherapy with Doxorubicin or Epirubicin and Cyclophosphamide. The addition of Olanzapine 10 mg is known to reduce acute and delayed CINV, however, Olanzapine 10 mg is associated with somnolence and fatigue. In this study, the proportion of complete response of CINV to Olanzapine 5 mg was compared to Olanzapine 10 mg that was added to standard antiemetic regime. Methods: This was an open-label randomized clinical trial comparing the effectiveness of Olanzapine 5 mg against Olanzapine 10 mg in preventing acute and delayed phases of CINV. This study was conducted at a tertiary care cancer centre in India between 2020 – 2021. The sample size was calculated 118 subjects that were allocated with simple randomization method. The primary outcome of CINV in acute and delayed phase were collected using the Multinational Association for Supportive Care in Cancer (MASCC) Antiemesis Tool; the intensity of symptoms of nausea, retching and vomiting were assessed using the Rhodes index; the overall patient experience was assessed using the Edmonton Symptom Assessment Scale. The comparison of proportions of CINV in the two groups were compared using Chi-square test while intensity scores were compared using Wilcoxon rank-sum test. All p values <0.05 were considered significant. Ethics approval was obtained from the Institutional Review Board. The trial was registered at the Clinical Trials Registry of India. Results: Of the118 patients randomized, 57 received Olanzapine 5 mg and 61 received Olanzapine 10 mg in addition to standard antiemetic regime. The overall complete response rate was 54.2% in acute phase and 72.0% in delayed phase. The complete response rate of Olanzapine 5 mg was 57.9% in acute phase and 91.2% in delayed phase. The complete response rate of Olanzapine 10 mg was 50.9% in acute phase and 54.1% in delayed phase. The nausea intensity score on day 5 after chemotherapy was significantly lower among those receiving Olanzapine 10 mg. However, there were no difference in the reduction retching and overall symptoms between the groups. In terms of side effect profile, those receiving Olanzapine 10 mg reported significantly higher grades of somnolence and fatigue. There was no difference in the occurrence of pain, depression, anxiety, shortness of beath, appetite and wellbeing. Conclusions: There was no difference in the effectiveness of Olanzapine 5 mg compared to Olanzapine 10 mg in the prevention of CINV. However, Olanzapine 10 mg was associated with higher proportions of somnolence and fatigue. Therefore, it is recommended that Olanzapine 5 mg may replace current antiemetic regimes that contain Olanzapine 10 mg. Clinical trial information: CTRI/2020/01/023076.