Background

Selenium is an essential trace mineral found in soil, water, and some foods. It is an important factor in many body processes.

Selenium increases antioxidant effects in the body. Crab, fish, poultry, and wheat are generally good food sources. The amount of selenium in soil varies, and foods grown in different soils have different selenium levels. The Eastern Coastal Plain and Pacific Northwest have the lowest selenium levels in the US.

People commonly use selenium for selenium deficiency and to reduce the risk for high blood pressure during pregnancy. It is also used for prostate cancer, complications from statin drugs, abnormal cholesterol levels, cataracts, and many other conditions, but there is no good scientific evidence to support most of these uses. There is also no good evidence to support using selenium for COVID-19.

Safety Safety definitions

When taken by mouth: Selenium is likely safe when taken in doses less than 400 mcg daily, short-term. But selenium is possibly unsafe when taken in high doses or for a long time. Taking doses above 400 mcg daily can increase the risk of developing selenium toxicity. Taking lower doses for a long time can increase the risk of developing diabetes.

Selenium can cause stomach discomfort, headache, and rash. High doses can cause hair loss, fatigue, nausea, vomiting, and weight loss. Extremely high doses can lead to organ failure and death.

Special Precautions & Warnings:

Pregnancy and breast-feeding: Selenium is possibly safe when used short-term in amounts that are not above 400 mcg daily. Selenium is possibly unsafe when taken by mouth in doses above 400 mcg daily. This dose might cause toxicity.

Children: Selenium is possibly safe when taken by mouth appropriately. Selenium seems to be safe when used short-term in doses below 45 mcg daily for infants up to age 6 months, 60 mcg daily for infants 7-12 months, 90 mcg daily for children 1-3 years, 150 mcg daily for children 4-8 years, 280 mcg daily for children 9-13 years, and 400 mcg daily for children age 14 years and older.

Autoimmune diseases: Selenium might stimulate the immune system. People with autoimmune diseases such as multiple sclerosis, systemic lupus erythematosus (SLE), rheumatoid arthritis (RA), and other should avoid taking selenium supplements.

Hemodialysis: Blood levels of selenium can be low in people on hemodialysis. Selenium supplements might be needed for some people.

Under-active thyroid (hypothyroidism): Taking selenium can worsen hypothyroidism, especially in people with iodine deficiency. In this case, you should take iodine along with selenium. Check with your healthcare provider before taking selenium supplements.

Fertility problems in males: Selenium might decrease the ability of sperm to move, which could reduce fertility.

Skin cancer: In people who have had nonmelanoma skin cancer, long-term use of selenium supplements might slightly increase the risk of cancer returning. Avoid long-term use of selenium supplements if you have ever had skin cancer.

Surgery: Selenium might increase the risk of bleeding during and after surgery. Stop taking selenium at least 2 weeks before a scheduled surgery.

Effectiveness

Dosing & administration

Selenium is an essential trace mineral found in foods, including crab, fish, poultry, and wheat. The amount of selenium in soil varies, so foods grown in different soils have different selenium levels. The amount that should be consumed on a daily basis is called the recommended dietary allowance (RDA). The RDA is 55 mcg daily for all people 19 years and older. While pregnant, the RDA is 60 mcg daily. While breastfeeding, the RDA is 70 mcg daily. In children, the RDA depends on age.

Interactions with pharmaceuticals

Birth control pills (Contraceptive drugs)

Interaction Rating=Minor Be watchful with this combination.

Taking birth control pills might increase blood levels of selenium. But it's not clear if this is a real concern.

Medications that decrease the immune system (Immunosuppressants)

Interaction Rating=Moderate Be cautious with this combination.

Selenium can increase the activity of the immune system. Some medications, such as those used after a transplant, decrease the activity of the immune system. Taking selenium along with these medications might decrease the effects of these medications.

Medications that slow blood clotting (Anticoagulant / Antiplatelet drugs)

Interaction Rating=Moderate Be cautious with this combination.

Selenium might slow blood clotting. Taking selenium along with medications that also slow blood clotting might increase the risk of bruising and bleeding.

Niacin

Interaction Rating=Moderate Be cautious with this combination.

Taking niacin along with the drug simvastatin can increase good cholesterol levels. Taking niacin plus simvastatin along with selenium and other antioxidants can decrease the effects of niacin and simvastatin on good cholesterol levels. It is unknown if selenium alone decreases the effects of niacin plus simvastatin on good cholesterol levels.

Sedative medications (Barbiturates)

Interaction Rating=Moderate Be cautious with this combination.

Selenium might reduce how quickly the body breaks down sedative medications. Taking selenium with these medications might increase the effects and side effects of these medications.

Warfarin (Coumadin)

Interaction Rating=Moderate Be cautious with this combination.

Selenium might slow blood clotting. Selenium might also increase the effects of warfarin in the body. Taking selenium along with warfarin might increase the chances of bruising and bleeding.

Interactions with herbs & supplements

Astragalus: Some species of astragalus contain large amounts of selenium, especially when grown in selenium-rich soils. Taking products made from these plants along with selenium supplements could cause selenium poisoning. But most astragalus supplements contain Astragalus membranaceus, which does not contain high levels of selenium.
Copper: Selenium might increase how quickly the body removes copper. Taking selenium might reduce copper levels in the body.
Herbs and supplements that might slow blood clotting: Selenium might slow blood clotting and increase the risk of bleeding. Taking it with other supplements with similar effects might increase the risk of bleeding in some people.
Niacin: Niacin can increase good cholesterol levels. Taking selenium along with beta-carotene, vitamin E, and vitamin C might decrease the effects of niacin on good cholesterol levels. It is not known if selenium alone decreases the effects of niacin on good cholesterol levels.
Omega-3 fatty acids: Taking selenium with omega-3 fatty acids might reduce how much selenium the body absorbs.
Vitamin C: Taking vitamin C might affect how much selenium the body absorbs from some supplements. But it's unlikely that this is a big concern.
Zinc: Zinc might make it more difficult for the body to absorb selenium from food.

Interactions with foods

Selenium is a nutrient commonly obtained from food. Following the ketogenic diet, which is a high-fat, low-carbohydrate diet, might lead to vitamin and mineral deficiencies, including selenium deficiency.