Adverse effects have rarely been observed in clinical trials, and when such effects were reported, their frequency was not higher than the one observed in the control groups treated with placebo. Consequently, most of the observed effects were not causally related to soy or to isoflavones.

Typical adverse events reported in clinical trials are mild gastrointestinal complaints which usually disappear even with continued intake of the soy or isoflavone preparation.

Severe adverse effects have never been observed to date – which includes hormonal, estrogen-like adverse effects (see also the contributions to hormonal safety).

Soy features a very high safety of application. Concerns related to safety have been essentially worded regarding three topics:

• - Potential allergies against soy proteins;
• - The safety of application in women with breast cancer, and
• - The impact of isoflavones on the thyroidal gland in patients with thyroidal deficiency.

In all these cases most recent data allows calling off the alarm: The application of soy is safe. Not only is soy only rarely causing allergies, but it is also no risk factor for breast cancer and the thyroidal gland. In the contrary, the observations related to hormone-sensitive cancers such as tumours of the breast, the uterus or (in men) the prostate rather point to protective effects in consumers. This protection does not only relate to the mentioned hormone-sensitive cancers, but also to lung cancer and colon cancer. In all cases regular consumers of isoflavone-containing food will most likely profit from a reduced cancer incidence.