Flaxseeds contain phenolic compounds called lignans and secolariciresinol diglucoside (SDG) is a major lignan with putative health benefits such as antioxidant and anticancer effects. The role of SDG and its metabolites such as enterolignans is gaining attention due to their mitigating effects against cancers especially prostate and breast cancer. Several epidemiological, in vitro and animal studies add evidence to this anti-cancer benefit of SDG. However, more research activities, especially clinical and pharmacokinetic studies in humans are required to corroborate this evidence. This review attempts to focus on the roles of SDG and its metabolites in preventing breast tumors, including an evaluation of potential mechanisms of action.