How much is too much selenium in a day?

November 17, 2025 •

2 min read

According to the National Institutes of Health, the tolerable upper intake level (UL) for selenium in adults is 400 micrograms (mcg) per day. Understanding this limit is crucial, as too much selenium in a day can lead to a condition called selenosis, which carries a range of adverse health effects.

Quick Summary

The Tolerable Upper Intake Level for selenium is 400 mcg daily for adults, a threshold that, if exceeded, can lead to toxicity known as selenosis with symptoms like hair loss and gastrointestinal issues. Most people meet their needs through diet, but high-dose supplements or specific foods, like Brazil nuts, can push intake into dangerous territory.

Key Points

Tolerable Upper Intake Level (UL): For adults, the safe upper limit for selenium is 400 micrograms (mcg) per day from all sources combined.
High-Dose Risks: Consuming selenium above the 400 mcg UL can cause selenosis, characterized by symptoms like hair loss, brittle nails, and gastrointestinal issues.
Diet vs. Supplements: Overdose is most often linked to high-dose supplements or excessive intake of specific foods like Brazil nuts, not a balanced diet.
Signs of Toxicity: Early indicators of too much selenium include a metallic taste, garlic-scented breath, fatigue, and gastrointestinal upset.
Serious Complications: Chronic and severe selenosis can lead to kidney and heart failure, and neurological problems.
Consult a Professional: Always talk to a doctor before starting selenium supplements, as they can have risks and interact with medications.

The Importance of Selenium and Why More Isn't Always Better

Selenium is an essential trace mineral involved in metabolism, thyroid function, and antioxidant defense. Selenoproteins, which contain selenium, are powerful antioxidants.

The Tolerable Upper Intake Level (UL)

The Tolerable Upper Intake Level (UL) for selenium in healthy adults is 400 micrograms (mcg) daily, encompassing intake from all sources. Exceeding this consistently increases the risk of adverse effects. The Recommended Dietary Allowance (RDA) for adults is much lower at 55 mcg per day. Toxicity is rare from diet alone and usually results from excessive supplementation or consuming large quantities of specific foods.

Signs and Symptoms of Selenosis

Selenosis, or chronic selenium toxicity, develops from prolonged high intake. Symptoms vary and can be subtle initially.

Garlic Breath: A metallic taste and garlic-like breath are early indicators.
Hair and Nail Changes: Hair loss and brittle or discolored nails are common.
Gastrointestinal Distress: Nausea, vomiting, diarrhea, and abdominal pain can occur.
Neurological Problems: Severe cases can involve fatigue, irritability, tremors, and nerve damage.
Organ Failure: Extreme, long-term exposure can lead to kidney or heart failure and, rarely, death.

High-Risk Sources of Selenium Overdose

A balanced diet typically provides enough selenium, making supplements often unnecessary. However, certain sources increase toxicity risk. Brazil nuts, for instance, can be extremely high in selenium, with a single nut possibly containing up to 95 mcg. Regular, high-dose supplementation is another significant risk factor.

Comparison: Selenium Deficiency vs. Excess

Both insufficient and excessive selenium intake are detrimental.


Feature	Selenium Deficiency	Selenium Excess (Selenosis)
Cause	Low dietary intake, low-selenium soil, malabsorption	Over-supplementation, high intake of certain foods, environmental exposure
Early Signs	Weak immune system, fatigue, muscle weakness	Metallic taste, garlic breath, fatigue
Characteristic Symptoms	Keshan disease (heart condition), Kashin-Beck disease (joint condition)	Hair loss, brittle nails, skin issues
Gastrointestinal Issues	Less frequent	Nausea, vomiting, diarrhea, abdominal pain
Neurological Impact	Increased risk of neurodegenerative disease	Tremors, nerve damage, irritability
Long-Term Effects	Poor thyroid function, increased heart disease risk	Potential organ failure, increased risk of diabetes and some cancers

Conclusion: A Fine Balance

The 400 mcg UL for adults protects against selenosis. Most people get sufficient selenium from a varied diet without supplements. High-dose supplements should be approached with caution due to potential links to conditions like type 2 diabetes and certain skin cancers. Always consult a healthcare provider before taking supplements, especially with existing health conditions.

For more detailed information on nutrient guidelines, consult the National Institutes of Health (NIH) Office of Dietary Supplements website.

Frequently Asked Questions

The Tolerable Upper Intake Level (UL) for selenium in adults is 400 micrograms (mcg) per day. This level includes all sources, such as food, beverages, and supplements. Exceeding this consistently can lead to toxicity.

It is very rare to get too much selenium from a normal, balanced diet. However, consuming very high amounts of certain foods, such as Brazil nuts, can lead to excessive intake. For example, a single Brazil nut can contain up to 95 mcg of selenium.

Early symptoms of selenium toxicity, or selenosis, often include a metallic taste in the mouth, breath that smells like garlic, fatigue, and mild skin rashes. Hair loss and brittle nails can also occur.

In severe cases, a chronic overdose of selenium can result in serious health problems like neurological disorders, kidney failure, heart problems, and, in extremely rare instances, can be fatal.

A daily supplemental dose of 200 mcg is generally considered safe for most healthy adults, as it is below the 400 mcg upper limit. However, for many people with a balanced diet, supplementation is not necessary, and it is best to consult a healthcare provider.

Yes, selenium can interact with certain medications. For example, it might slow blood clotting, increasing the risk of bleeding when taken with anticoagulant or antiplatelet drugs. It can also interact with chemotherapy drugs and niacin.

Symptoms of selenosis typically start to resolve within about four weeks after the high selenium intake is stopped. However, the recovery time can vary depending on the severity of the toxicity.