Can flaxseed be used to prevent or treat breast cancer?

Flaxseed contains lignans, a type of phytoestrogen that can have a weak estrogenic effects but can also block estrogen receptors. Some types of breast cancer are called estrogen-receptor positive (ER+), because the tumor cells have estrogen receptors and can grow in response to estrogen. Because lignans can block estrogen receptors, a flaxseed supplement may slow the growth of certain types of breast cancer. Several studies have found an association between flaxseed supplementation and a decrease in the risk of breast cancer, a slowed growth rate of tumors, an increase in cancer cell death, and a reduction in mortality in people with breast cancer, especially in postmenopausal women and in ER+ tumors.^[1][2][3][4] However, these are observational studies, and clinical trials are needed to confirm the findings before flaxseed could be a recommended part of breast cancer treatment. In a randomized controlled trial, flaxseed was found to be safe, but it was not effective in reducing biomarkers of breast-cancer-cell proliferation.^[5] Furthermore, all treatments should be reviewed by a medical professional before starting flaxseed supplementation, because flaxseed can interact with some of the medications used to treat breast cancer, such as tamoxifen and trastuzumab.^[4][6]