Background Nonconformity to chemotherapy schedules is common in clinical practice. Multiple clinical studies have established the negative prognostic impact of dose delay on survival outcome. Objective This study investigated the prevalence and reason for chemotherapy schedule modifications of breast cancer patients. This study also investigated the impact of schedule modifications on overall survival (OS). Setting This retrospective cohort study was done among breast cancer patient receiving chemotherapy from 2013 to 2017 and patients were followed until 31 Dec 2018. Methods Medical records of patients with cancer were reviewed. Female patients over eighteen years old were included, with primary carcinoma of the breast, who received anthracycline or taxane based chemotherapy regime and completed more than two cycles of chemotherapy. Patients were categorized into three groups of (1) no schedule modification, (2) with schedule modification and (3) incomplete schedule. The Kaplan-Meier was used to test for survival differences in the univariate setting and Cox regression model was used in the multivariate setting. Main outcome measure Prevalence, overall survival rates and hazard ratio of three schedule group Results Among 171 patient who were included in the final analysis, 28 (16.4%) had no schedule modification, 118 (69.0%) with schedule modification and 25 (14.6%) had incomplete schedule with OS of 75.0%, 59.3% and 52.0% respectively. 94% (189) of all cycle rescheduling happened because of constitutional symptoms (70), for non-medical reasons (61) and blood/bone marrow toxicity (58). When compared to patients with no schedule modification, patients with schedule modification had a 2.34-times higher risk of death (HR 2.34, 95% CI 1.03-5.32; p = 0.043). Conclusion Nonconformity to the chemotherapy schedule is common in clinical practice because of treatment complications, patients' social schedule conflicts, and facility administrative reasons. Cumulative delays of ≥ 14 days are likely to have negative prognostic effect on patient survival. Thus, the duration of the delays between cycles should be reduced whenever possible to achieve the maximum chemotherapeutic benefit.