Background: Patients undergoing breast cancer treatment often report Symptoms of Cognitive Deficit (SCD). Many of them share their experiences on online forums, which contain millions of freely shared messages that can be used to analyze these SCD. Unfortunately, this data is unstructured, making it difficult to analyze. In this project we organize this data using methods from Big Data Science (BDS) and analyze it by creating a Decision Support System (DSS): an interface that can be used by patients and providers to understand how SCD are associated with specific types – hormonal only (HT), chemo only (CT), or both (CT/HT) – of breast cancer therapies. Methods: We collected 3.5 million unique messages from 20 unrestricted breast cancer forums that provide clinically relevant information. We next built custom ontologies for breast cancer treatments, SCD, and supportive therapies. Then, we created a DSS using methods from BDS, including topic modeling, information retrieval, and natural language processing to extract the relevant data from these messages. We also used token windows and co-occurrence-based algorithms to associate treatment with SCD and supportive therapies. To use this system, a user provides disease-related parameters and the treatment. The DSS then gives the percentage of messages discussing SCD for a similar cohort of patients and the percentage of messages that discuss supportive therapies for each of these SCD. Results: We found 15719 messages that had strong association of SCD with treatments. 3355 messages were from HT patients, 5740 messages were from CT patients, and 9095 messages were from CT/HT patients. Among HT, 28.18% patients taking aromatase inhibitors and 19.20% taking tamoxifen associated SCD to HT. Among CT, 35.26% patient receiving taxane containing chemo associated SCD to CT. SCD worsened during HT for CT/HT patients. Suggestive therapy: 80 messages found Vitamin B12 and B6 useful, 65 suggested Acetyle-L-Carnitine, and 50 suggested playing word games. Conclusions: Using methods from BDS, our DSS reliably associates SCD with HT, CT and CT/CT, and suggests supportive therapies. More research is needed to evaluate the role of supportive therapy for SCD.