Side effects from drinking soy milk include constipation, bloating and nausea, and it can cause allergic reactions including rash and itching, according to WebMD. Using soy dietary supplements in high doses may cause abnormal intrauterine tissue growth and may harm a baby's development when used as a medicine during pregnancy.
Lactating mothers should avoid taking large doses of the supplement, states WebMD. Infants consuming regular soy milk can develop nutrient deficiencies, and parents should not substitute it for infant formula, or give it to children with milk allergies. Patients with endometrial cancer should not use a soy supplement, and those with breast cancer or a history of breast cancer, bladder cancer or kidney stones should avoid it. People with asthma and hay fever may be allergic to soy hulls.