There was a Breast Cancer Support Network meeting tonight (they meet by Zoom every Monday, it's for people with non-metastatic breast cancer, Stages 1-3) where the guest was Dr. Laura Rhaney from Mayo. I specifically asked about flaxseed, and she said it's fine.

Also, Lisa Ellsworth, PA-C at Mayo, did a study on supplements and their effects on breast cancer. I've attached it; I found it to be very informative.