Background: Reduced relative dose intensity (RDI) of neoadjuvant chemotherapy (NACT) in patients with breast cancer may compromise treatment outcome and survival. We examined patient-related characteristics associated with treatment modifications and suboptimal RDI and tumour response in patients with breast cancer. Methods: In this observational study, electronic medical records were reviewed retrospectively for female patients with breast cancer scheduled for NACT at a university hospital in Denmark between 2017 and 2019. The RDI (ratio of delivered dose intensity in relation to standard dose intensity) was calculated. Multivariate logistic regression analyses examined associations of sociodemographics, general health and clinical cancer characteristics with dose reductions, dose delays, discontinuation of NACT and suboptimal RDI < 85%. Results: Among 122 included patients, 43%, 42% and 28% experienced dose reductions, dose delays ≥3 days and discontinuation, respectively. A total of 25% received an RDI < 85%. Comorbidity, taking long-term medications and being overweight were statistically significantly associated with treatment modifications, while age ≥ 65 years and comorbidity were associated with RDI < 85%. Around one third of all patients had radiologic (36%) or pathologic (35%) complete tumour response, with no statistically significant differences by RDI < or ≥85% irrespective of breast cancer subtype. Conclusions: While most patients had RDI ≥85%, still one out of four patients received an RDI < 85%. Further investigations of possible supportive care initiatives to improve patients’ treatment tolerability are needed, particularly among subgroups of older age or with comorbidity.