What happens when you take selenium every day?

October 30, 2025 •

4 min read

According to the National Institutes of Health, the Recommended Dietary Allowance (RDA) for selenium in adults is 55 micrograms (mcg) per day. This vital trace mineral supports the immune system, thyroid function, and acts as a powerful antioxidant, but what happens when you take selenium every day, especially via supplements?

Quick Summary

Taking selenium every day at appropriate dosages supports immune function, antioxidant defense, and thyroid health, but excessive intake can lead to toxicity, hair loss, and neurological issues. The health effects depend heavily on individual intake levels and overall selenium status.

Key Points

Daily Selenium Offers Benefits: Regular intake supports the immune system, acts as an antioxidant, and is vital for thyroid function.
Excessive Intake is Toxic: Over 400 mcg daily can lead to selenosis, with symptoms including hair loss, nausea, and nerve damage.
Risk Varies by Source: Supplementation carries a higher risk of toxicity than dietary intake, as it is easier to consume too much in a concentrated form.
Food is the Safest Source: Most individuals can meet their selenium needs through a balanced diet rich in selenium-containing foods.
Thyroid Patients May Benefit: Supplementation under medical guidance can help manage autoimmune thyroid conditions like Hashimoto's disease.
Long-Term High Doses Carry Risks: Studies have linked prolonged high-dose intake to an increased risk of type 2 diabetes and certain cancers in some populations.
Monitor Your Status: Checking plasma selenium levels can be helpful, especially when considering long-term supplementation.

The Dual Nature of Daily Selenium Intake

Selenium is a trace mineral crucial for human health, playing a key role in various physiological processes through selenoproteins, which have antioxidant and regulatory functions. Found in soil, its concentration in food varies geographically. For most people in countries with adequate soil selenium, a balanced diet provides sufficient intake. However, for some, daily supplementation might be a consideration. The impact of taking selenium every day is highly dependent on dosage and existing nutritional status.

What are the benefits of daily selenium intake?

Regular, moderate intake of selenium offers several health advantages, particularly for those with a deficiency. The benefits are primarily linked to the mineral's role in creating selenoproteins.

Powerful Antioxidant Defense: Selenium is integral to glutathione peroxidase enzymes, which protect cells from oxidative stress and free radical damage. Reducing oxidative stress is linked to a lower risk of chronic conditions like heart disease and certain cancers.
Supports Thyroid Health: The thyroid gland contains the highest concentration of selenium in the body. Selenium is essential for the production and regulation of thyroid hormones, and deficiency is linked to thyroid conditions. Some studies suggest supplementation can benefit those with autoimmune thyroiditis, like Hashimoto's disease.
Boosts Immune System Function: Selenium plays a critical role in immune system health by helping to regulate inflammation and enhance the body's immune response. Adequate levels are needed for the proper functioning of immune cells.
Supports Cognitive Function: Some research links lower selenium levels to an increased risk of age-related mental decline and neurological diseases like Alzheimer's. The mineral's antioxidant properties may help protect brain cells from damage.

What are the risks of excessive daily selenium intake?

While beneficial in moderation, selenium has a narrow therapeutic window, meaning the line between adequate and toxic intake is relatively small. Consuming more than the tolerable upper intake level (UL) of 400 mcg per day for adults can lead to a condition called selenosis.

Selenium Toxicity (Selenosis): Early signs of toxicity can include a garlic-like odor on the breath, a metallic taste, fatigue, and gastrointestinal upset such as nausea and diarrhea.
Hair and Nail Changes: One of the most noticeable chronic effects is hair loss and brittle or discolored nails.
Neurological Symptoms: In severe cases of toxicity, neurological issues can occur, including tremors, nerve damage (peripheral neuropathy), irritability, and mood swings.
Increased Risk of Chronic Disease: Long-term, high-dose supplementation has been linked to an increased risk of type 2 diabetes and may also affect cholesterol levels.
Potential for Organ Damage: Extremely high doses over prolonged periods can lead to more serious health problems, including kidney failure and heart issues.

Comparison of Selenium Intake: Diet vs. Supplements

It is important to differentiate between getting selenium from food and taking it in supplement form. The risks associated with high intake are primarily linked to supplement use, where it's easier to consume excessive amounts.


Feature	Dietary Selenium Intake	Supplemental Selenium Intake
Source	Naturally occurring in foods like Brazil nuts, seafood, meat, and eggs.	Concentrated doses in pills, capsules, or fortified yeast.
Risk of Toxicity	Extremely low. It is very difficult to reach toxic levels through food alone, though eating an excessive number of Brazil nuts is an exception.	Higher risk, as dosages can be high and consumption is less regulated than with natural food sources.
Absorption	Often absorbed in a balanced, more bioavailable form alongside other nutrients.	Varies by form (e.g., selenomethionine vs. selenite), with bioavailability potentially different from natural sources.
Recommended Approach	Most health experts recommend a food-first approach to meet the RDA.	Recommended for confirmed deficiencies or for individuals in areas with low soil selenium levels.

Long-Term Effects and Considerations

For individuals with a documented deficiency or specific health conditions like autoimmune thyroiditis, daily selenium supplementation under medical supervision can be beneficial. However, long-term, high-dose supplementation for healthy individuals is not consistently supported by research and carries risks. Some studies have found little to no benefit for heart disease prevention in those with sufficient levels. Monitoring plasma selenium levels is essential when considering long-term use, as it can help ensure intake remains within a safe range.

Conclusion: The Importance of Balance

Taking selenium every day is a practice that offers distinct benefits but comes with significant risks if not managed properly. While daily intake at or slightly above the RDA is essential for optimal immune, antioxidant, and thyroid function, consistently exceeding the 400 mcg daily UL can have detrimental effects, leading to selenosis and other health problems. The source of selenium also matters, with a food-first approach being the safest and most recommended method for most people. Supplements should be considered primarily for correcting a confirmed deficiency under the guidance of a healthcare provider. As with many nutrients, the key to a healthy outcome with daily selenium intake lies in finding the right balance for your body's specific needs, avoiding the pitfalls of both deficiency and excess.

Frequently Asked Questions

Taking selenium every day is safe as long as your intake stays below the 400 mcg Tolerable Upper Intake Level (UL) for adults. Most adults need around 55 mcg daily, which is often met through diet, making supplementation only necessary for those with a confirmed deficiency.

The initial signs of excessive selenium intake often include a garlic-like odor on the breath, a metallic taste in the mouth, fatigue, gastrointestinal upset like nausea and diarrhea, and hair loss or brittle nails.

The onset of side effects from excessive selenium can vary. With extremely high doses, symptoms can appear within a week, while chronic selenosis from long-term overuse typically develops over time.

Individuals with a history of certain types of skin cancer (like squamous cell carcinoma), autoimmune diseases, and those with certain medical conditions should consult a healthcare provider before taking supplements. People who are not deficient and those taking certain medications should also be cautious.

For adults, the tolerable upper intake level (UL) for selenium from all sources is 400 micrograms (mcg) per day. Consistently exceeding this amount increases the risk of toxicity (selenosis).

For individuals with autoimmune thyroid conditions like Hashimoto's disease, daily selenium supplementation under a doctor's supervision has been shown to potentially reduce antibody levels and improve symptoms. However, excessive amounts can harm the thyroid.

Yes, for most healthy individuals, obtaining selenium from dietary sources is safer. It is very difficult to consume toxic levels of selenium through food alone, unlike with concentrated supplements, which carry a higher risk of overdose.