Soy beans and other foods containing soy, such as tofu, tempeh and soy milk, are rich sources of phyto-oestrogens. Phyto-oestrogens are naturally occurring substances found in plant foods that act like the hormone oestrogen. There is some evidence that phyto-oestrogens may stimulate the growth of existing hormone dependent cancers. For example, it is not clear whether a diet high in phyto-oestrogens is safe for women who have breast cancer, particularly women with oestrogen receptor positive breast cancer who may be treated with Tamoxifen. The evidence of the impact of soy on Tamoxifen is inconsistent. On the other side of the coin, there is limited evidence suggesting that soy foods may lower the risk of prostate and stomach cancer. There has also been some research suggesting soy foods can reduce the risk of cancer of the pharynx, oesophagus, pancreas, breast and endometrium, however the findings are not clear or consistent enough for us to draw any conclusions. There is no known association between soy foods and the risk of other cancers. Cancer Council supports the consumption of soy foods in the diet. Current evidence shows moderate consumption of soy foods, as part of an overall healthy diet, is unlikely to have any harmful effects. However, we do not recommend use of supplements such as soy protein isolates or isoflavone capsules for healthy men and women to prevent cancer. We also advise women who have had breast cancer not to take soy or phyto-oestrogen supplements, nor suddenly increase the amount of soy phytoestrogens in the diet.