Timing of phytoestrogen use is important. Genistein makes some cells more sensitive to radio-therapy.
Though research is still ongoing, some recent studies have indicated that soy's phytoestrogens could be contributive factors in some forms of breast cancer, penile birth defects, and infantile leukemia.
Some studies have raised the concern that genistein might increase the risk of leukemia, because it inhibits the enzyme topoisomerase which results in double strand DNA breaks, which are, in turn, mutagenic. Some cancer patients whose chemotherapy drugs inhibited topoisomerase later developed leukemia. An animal study suggested that GEN may not be safe for postmenopausal women with estrogen-dependent breast cancer.
Regardless, soy's phytoestrogens, or isoflavones, have been definitely shown to depress thyroid function and to cause infertility in every animal species studied so far. Genistein's chief method of activity is as a tyrosine kinase inhibitor. Tyrosine kinases are less widespread than their ser/thr counterparts but implicated in almost all cell growth and proliferation signal cascades. Genistein has been used to selectively target pre B-cells via conjugation with an antibody. This highly successful study in mice has promising benefits for future chemotherapy

Effects in males

Isoflavones can act like estrogen, stimulating development and maintenance of female characteristics or they can block cells from using cousins of estrogen. In vitro studies have proven genistein to induce apoptosis of testicular cells at certain levels, thus raising concerns about effects it could have on male fertility.