Background

Soy (Glycine max) comes from soybeans. The beans are a legume that come from China. They can be processed into soy protein, soy milk, or soy fiber.

Soy contains isoflavones which are changed in the body to phytoestrogens. Phytoestrogen molecules are similar in chemical structure to the hormone estrogen. In some cases, these phytoestrogens can mimic the effects of estrogen. In other cases, these phytoestrogens can block the effects of estrogen.

Soy is used for high cholesterol, high blood pressure, heart disease, diabetes, symptoms of menopause, and premenstrual syndrome (PMS). It is also used for many other conditions, but there is no good scientific evidence to support many of these uses.

Safety Safety definitions

When taken by mouth: Soy is commonly consumed in foods. Dietary supplements containing soy extracts are possibly safe when used for up to 6 months. 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.

Special Precautions & Warnings:

Pregnancy: Soy is commonly consumed in foods. However, soy is possibly unsafe when used in the larger amounts found in medicine when pregnant. Higher doses during pregnancy might harm development of the baby.

Breast-feeding: There isn't enough reliable information to know if soy is safe to use in the larger amounts found in medicine when breast-feeding. Stay on the safe side and stick to food amounts.

Children: Soy is commonly consumed in foods. Giving infants soy formula doesn't seem to cause health or reproductive problems later in life. But soy milk that is not designed for infants should not be used as a substitute for infant formula. Regular soy milk could lead to nutrient deficiencies.

Soy is possibly unsafe when used as an alternative to cow's milk in children who are allergic to cow's milk. Although soy protein-based infant formulas are often promoted for children with milk allergy, these children are often allergic to soy as well.

Don't give children soy in amounts larger than what is found in food or formula. Researchers don't know whether soy is safe for children at higher doses.

Breast cancer: The effects of soy in people with breast cancer are unclear. Because there isn't enough reliable information about the effects of soy in females with breast cancer, a history of breast cancer, or a family history of breast cancer, it's best to avoid using soy supplements until more is known.

Allergy to peanuts and related plants: Soy might cause serious allergic reactions in people who are allergic to peanuts and other members of the Fabaceae plant family.

Under-active thyroid (hypothyroidism): Some people with this condition also have low iodine levels. Taking soy might make this condition worse in people who have low iodine levels.

Kidney failure: Soy contains a chemical called phytoestrogens. People with kidney failure who use soy products might have blood levels of phytoestrogens become too high. If you have kidney failure, avoid taking large amounts of soy.

Kidney stones: Soy products might increase the risk of kidney stones. Soy products contain large amounts of chemicals called oxalates. Oxalates are the main ingredient in kidney stones. If you have a history of kidney stones, avoid taking large amounts of soy.

Milk allergy: Children who are very allergic to cow's milk might also be sensitive to soy products. Use soy products with caution.

Bladder cancer: Soy products might increase the chance of getting bladder cancer. Avoid soy foods if you have bladder cancer or are at high risk of getting it.

Effectiveness

Dosing & administration

Soy foods are available in many different forms, including tofu, miso, and soy milk. Soy protein products have most often been used by adults in doses of 40 grams by mouth daily for up to 5 months. The active ingredients in soy, called soy isoflavones, have been used in supplements in doses of 120 mg by mouth daily for up to 6 months.

Soy is also used in topical products such as gels and moisturizers. Speak with a healthcare provider to find out what type of product and dose might be best for a specific condition.

Interactions with pharmaceuticals

Antibiotic drugs

Interaction Rating=Minor Be watchful with this combination.

Antibiotics are used to reduce harmful bacteria in the body. Antibiotics can also reduce friendly bacteria in the intestines. Friendly bacteria in the intestines help to convert soy to its active form. By reducing the number of bacteria, antibiotics might decrease the effects of soy. But it is too soon to know if this interaction is a big concern.

Caffeine

Interaction Rating=Moderate Be cautious with this combination.

Soy contains the chemical genistein. Genistein might slow down how quickly the body gets rid of caffeine. This might increase the effects of caffeine.

Estrogens

Interaction Rating=Moderate Be cautious with this combination.

Large amounts of soy might have some of the same effects as estrogen. Taking soy along with estrogen pills might decrease the effects of estrogen pills.

Levothyroxine (Synthroid, others)

Interaction Rating=Moderate Be cautious with this combination.

Levothyroxine is used for low thyroid function. Soy seems to decrease how much levothyroxine is absorbed by the body in infants, but not adults. This might decrease the effects of levothyroxine in infants. The dose of levothyroxine may need to be adjusted if soy is being used regularly, such as in soy-based formulas. Otherwise, take levothyroxine and soy at least 4 hours apart.

Medications for depression (MAOIs)

Interaction Rating=Major Do not take this combination.

Fermented soy products such as tofu and soy sauce contain tyramine. Tyramine is a naturally occurring chemical that is involved in blood pressure regulation. MAOIs can decrease how quickly the body breaks down tyramine. Consuming more than 6 mg of tyramine while taking one of these medications can increase the risk of serious side effects, such as very high blood pressure. If you take one of these medications, avoid fermented soy products that contain high amounts of tyramine.

Some common MAOIs include phenelzine (Nardil), selegiline (Zelapar), and tranylcypromine (Parnate).

Medications for diabetes (Antidiabetes drugs)

Interaction Rating=Moderate Be cautious with this combination.

Soy might lower blood sugar levels. Taking soy along with diabetes medications might cause blood sugar to drop too low. Monitor your blood sugar closely.

Medications for high blood pressure (Antihypertensive drugs)

Interaction Rating=Moderate Be cautious with this combination.

Soy might lower blood pressure. Taking soy along with medications that lower blood pressure might cause blood pressure to go too low. Monitor your blood pressure closely.

Progesterone

Interaction Rating=Moderate Be cautious with this combination.

Some research shows that taking soy milk along with progesterone might increase bone loss in females with osteoporosis.

Tamoxifen (Nolvadex)

Interaction Rating=Moderate Be cautious with this combination.

Tamoxifen affects the activity of estrogen in the body. Soy seems to also affect estrogen activity in the body. Taking soy with tamoxifen might change the effects of tamoxifen. Speak with a healthcare provider if you are taking tamoxifen.

Warfarin (Coumadin)

Interaction Rating=Moderate Be cautious with this combination.

Warfarin is used to slow blood clotting. Soy has been reported to decrease the effects of warfarin. This might increase the risk of clotting. Be sure to have your blood checked regularly. The dose of your warfarin might need to be changed.

Water pills (Diuretic drugs)

Interaction Rating=Moderate Be cautious with this combination.

Soy can increase urine production. The effects seem to be similar to those of "water pills." Taking soy along with "water pills" might increase the risk for side effects.

Interactions with herbs & supplements

Caffeine-containing herbs and supplements: Soy contains an ingredient that might slow down the removal of caffeine from the body. Taking soy with caffeine-containing herbs and supplements might increase the effects and side effects of caffeine. Examples of supplements that contain caffeine include black tea, coffee, green tea, guarana, and yerba mate.
Green tea: Green tea contains molecules called catechins. Many of the health benefits of green tea are thought to be due to these catechins. Taking soy protein along with green tea reduces how much catechins the body absorbs. Eating soy while drinking green tea might reduce the effects of green tea. Separate dosing of green tea and soy protein by at least three hours to avoid this interaction.
Herbs and supplements that might lower blood pressure: Soy might lower blood pressure. Taking it with other supplements that have the same effect might cause blood pressure to drop too much. Examples of supplements with this effect include andrographis, casein peptides, L-arginine, niacin, and stinging nettle.
Herbs and supplements that might lower blood sugar: Soy might lower blood sugar. Taking it with other supplements with similar effects might lower blood sugar too much. Examples of supplements with this effect include aloe, bitter melon, cassia cinnamon, chromium, and prickly pear cactus.
Iron: Soy might increase or decrease how much iron the body absorbs from food. But it isn't clear if this is a big concern.
Manganese: Soy might decrease how much manganese the body absorbs from food. But it isn't clear if this is a big concern.
Zinc: Soy might decrease how much zinc the body absorbs from food. But it isn't clear if this is a big concern.

Interactions with foods

Taking soy protein while drinking green tea might reduce the beneficial effects of green tea. To avoid this interaction, drink green tea at least 3 hours apart from consuming soy protein.

Soy might also change how much iron, manganese, or zinc is absorbed from foods. But this probably isn't a big concern for people who get enough of these nutrients in the diet.