Abstract Background To describe reasons for deviations from planned chemotherapy treatments in women with nonmetastatic breast cancer that contribute to less-than-planned receipt of chemotherapy. Methods Electronic medical records for patients receiving chemotherapy were reviewed for adverse events and treatment modifications. Log-binomial regression models were used to estimate relative risks (RRs) with 95% CIs to examine associations between chemotherapy modifications, patient characteristics, and treatment modalities. Results Delays in chemotherapy initiation (7%) were for surgical complications (58%), personal reasons (16%), and other (26%; port malfunction, infections, and obtaining extra imaging). Delays during chemotherapy (38%) were for infections (20%), neutropenia (13%), and personal reasons (13%). Dose reductions (38%) were for neuropathy (36%), unknown causes (9%), anemia (9%), and neutropenia (8%). Early treatment discontinuations (23%) were for neuropathy (29%). Patients receiving paclitaxel/nab-paclitaxel (RR 2.05; 95% CI, 1.47-2.87) and an anthracycline (RR 1.89; 95% CI, 1.39-2.57) reported more dose delays during chemotherapy. Black race (RR 1.46; 95% CI, 1.07-2.00), stage 3 (RR 1.79; 95% CI, 1.09-2.93), and paclitaxel/nab-paclitaxel receipt (RR 1.39; 95% CI, 1.02-1.90) increased the likelihood of dose reduction. Both Black race (RR 2.06; 95% CI, 1.35-3.15) and receipt of paclitaxel/nab-paclitaxel (RR 1.93; 95% CI, 1.19-3.13) increased the likelihood of early discontinuation. Patients receiving anthracyclines had higher rates of hospitalizations during chemotherapy (RR: 1.79; 95% CI, 1.11-2.89). Conclusion Toxicities are the most common reason for treatment modifications and need close monitoring in high-risk groups for timely intervention. Dose reductions and early treatment discontinuations occurred more for Black patients and need further study.