Your question says metastatic BC. I certainly would be asking more questions of my doctor than about acid from tamoxifen which could certainly be treated with a host of proton pump inhibitors available both as prescribed and over the counter. Metastatic BC is a killer left untreated. I would be asking tons of questions, and sooner than later. Do you have an appt. set up? Here are some questions I would start with. Where are your Mets? What is he doing to treat them? How is he planning to monitor them? What are my expectations of successful treatment?