Phytoestrogens are plant-derived compounds with estrogen-like activity, found in foods such as soy, flaxseed, and whole grains. Their use in supporting cancer treatment, particularly for reducing side effects associated with hormone therapies in breast and prostate cancer, has some basis in scientific research, but the evidence remains limited and sometimes controversial.

Several studies have explored phytoestrogens' potential to alleviate menopausal symptoms (such as hot flashes) caused by cancer treatments that lower estrogen levels. Isoflavones, a class of phytoestrogens, have shown mild effectiveness in reducing the frequency and severity of hot flashes in some clinical trials, though results are inconsistent. Some data also suggest possible benefits for bone health and cardiovascular risk reduction in cancer survivors.

However, there is also concern about phytoestrogens' estrogenic effects, especially in hormone-sensitive cancers like breast cancer. Some studies indicate that phytoestrogens might compete with endogenous estrogens for receptor binding, potentially exerting anti-estrogenic effects, while others raise the possibility that they could stimulate cancer cell growth under certain conditions.

Overall, while phytoestrogens are used and studied for reducing certain side effects in cancer treatment, the supporting evidence is moderate (rated 2/5) and not unequivocal. Clinical guidelines generally advise caution, and patients are encouraged to consult their healthcare providers before using phytoestrogen supplements during cancer therapy.