For some time, there has been widespread concern about the safety of soy products for breast cancer patients. This concern stems from the fact that soy products contain high levels of soy isoflavones, a phytoestrogen that produces estrogen-like effects in the body and may increase the risk of recurrence in breast cancer patients. In addition, because soy isoflavones and tamoxifen, a drug used to prevent recurrence and metastasis in breast cancer patients, both work in combination with the same structure in the body, they may compete with each other, thus affecting the efficacy of the drug. To investigate whether soy isoflavones increase the risk of recurrence and death from breast cancer, a global multi-institutional survey was launched to evaluate the consumption of soy products among 18,312 patients aged 20 to 83 years with confirmed breast cancer using a questionnaire. Of these, 16,048 consumed soy isoflavones (average daily consumption, 3.2 mg for US women and 45.9 mg for Shanghai women), tofu, and soy milk. Those who consumed more than 23 mg of soy isoflavones per day had a 9% lower risk of death compared to those who consumed lower doses of soy isoflavones (less than 0.48 mg per day). This result suggests that consumption of soy isoflavones is beneficial for breast cancer patients.