Creatine and Cognitive Health in Patients With Early-stage Breast Cancer During and After Chemotherapy Treatment

The goal of this clinical trial is to evaluate whether oral creatine supplementation can prevent chemotherapy-related cognitive impairment in patients with early-stage breast cancer undergoing neoadjuvant or adjuvant chemotherapy. The main questions it aims to answer are: * Does creatine supplementation reduce the occurrence and severity of chemotherapy-related cognitive impairment, as measured by patient-reported cognitive function? * Does creatine supplementation preserve objective cognitive performance compared with placebo during and after chemotherapy? Researchers will compare creatine supplementation (5 g/day) with placebo to assess differences in cognitive outcomes, safety, and exploratory biological markers. Participants will: * Receive oral creatine or placebo starting 7 days before chemotherapy and continuing until 12 months after chemotherapy completion * Complete patient-reported cognitive assessments and objective neuropsychological tests at predefined time points * Undergo clinical follow-up for safety and oncologic outcomes * Provide blood samples for biomarker analyses and stool samples for gut microbiota assessment * Undergo brain magnetic resonance imaging at selected study visits