The Federal Institute for Risk Assessment (BfR) in Germany has said the long-term intake of supplements containing isoflavones carries a risk to menopausal women.
The group has evaluated a number of scientific studies after receiving complaints about side effects, and concluded the oestrogen like effects of the bioactive plant substance could promote the development of cancer.
However, a spokesperson for industry group the Health Food Manufacturers Association (HFMA), based in the UK, said it will ask its scientific advisers to look at the 23-page review by BfR.
The assessment could be seen as blow to the ingredient, which has won praise in scientific studies linking it having a positive effect on aiding men at risk of prostate cancer and reducing diarrhoea in infants. However, the issue of its effectiveness on menopausal women has not been so clear cut.
Soy isoflavones are naturally occurring oestrogen-like compounds, and supplements are currently marketed as a way of reducing symptoms of the menopause and offer an alternative to hormone replacement therapy.
Conflicting reports however have clouded the picture about the beneficial effects of soy isoflavones, with some studies indicating that breast cancer cells in mice were stimulated by the isoflavones. Population studies have shown that women with a high-soy diet generally have lower rates of breast cancer.
The BfR's review says that there is not enough data to show that foods containing soy and red clover isoflavones are safe.
"The necessary long-term studies, to prove the safety of isoflavone-containing products, are not available," the institute said.
The regulator said: "BfR is of the opinion that the toxicological risks regarding the hormonal situation of female users are to be viewed more critically."
BfR concludes there is a need for action and suggests raising awareness amongst women with breast cancer, or those at risk of developing the condition, as to the effects of isoflavones.
The institute said supplements with isolated isoflavones have been available on the market as an alternative to the prescribed hormone replacement therapy for menopausal complaints.
It found that toxicological studies on isoflavones administered at high doses impair the functioning of the thyroid gland and can change mammary gland tissue.
"It cannot be ruled out that these estrogen-like effects could promote the development of breast cancer," BfR said.
The report even goes further and blasts isoflavones advantageous health effects on heart, bones and breasts, which it deems as "not sufficiently scientifically substantiated."
The group advises against the long-term intake of supplements with isoflavones. Between 2002 and 2006, 21 complaints were received relating to adverse side effects of isoflavone-containing products.
Side-effects included itching, eczema, nausea, dizziness, abdominal pain, skin rash and sweating.