The most common side effects of soy are digestive upsets, such as constipation and diarrhea. Soy may alter thyroid function in people who are deficient in iodine. Current evidence indicates that it’s safe for women who have had breast cancer or who are at risk for breast cancer to eat soy foods.

Soy can cause some mild stomach and intestinal side effects such as constipation, bloating, and nausea. It can also cause allergic reactions involving rash, itching, and breathing problems in some people.