Can Selenium Supplements Cause Diarrhea? Understanding the Side Effects

November 17, 2025 •

4 min read

In a 2010 outbreak, a mislabeled supplement containing 200 times the intended selenium concentration led to widespread toxicity, with 78% of those affected reporting diarrhea. This illustrates that yes, high-dose selenium supplements can cause diarrhea, though it is not a common side effect when taken within recommended limits.

Quick Summary

Excessive selenium intake from supplements can lead to gastrointestinal upset, including diarrhea, nausea, and vomiting. This symptom is a sign of selenosis, or selenium toxicity, and occurs primarily with high doses exceeding the Tolerable Upper Intake Level (UL) of 400 mcg per day for adults.

Key Points

Excessive Intake is the Cause: Diarrhea from selenium supplements is a sign of toxicity (selenosis), which occurs when the daily intake far exceeds safe limits, particularly over 400 mcg per day for adults.
Gastrointestinal Upset is Common: High doses of selenium frequently cause nausea, vomiting, and diarrhea as the body tries to expel the excess mineral.
Know the Signs of Toxicity: Be aware of other selenosis symptoms, such as a garlic-like breath odor, hair loss, and brittle nails.
Consider the Supplement Form: Organic forms of selenium, like selenomethionine, are generally considered safer and less toxic than inorganic forms at similar doses.
Stop and Consult a Doctor: If you experience diarrhea or other adverse effects, immediately stop taking the supplement and seek medical advice to address the potential toxicity.
Check Your Total Intake: Take into account all sources of selenium, including high-selenium foods like Brazil nuts, to ensure you are not exceeding the safe upper limit.

The Link Between Selenium and Diarrhea

Diarrhea is a known symptom of excessive selenium intake, or selenium toxicity (selenosis). While selenium is an essential trace mineral vital for functions like thyroid hormone metabolism and immune response, the window between a beneficial dose and a toxic one is relatively narrow. When intake from supplements far exceeds the body's needs, particularly the 400 mcg daily Tolerable Upper Intake Level (UL) for adults, it can overwhelm the body's metabolic processes. This overload can cause a number of adverse effects, with gastrointestinal disturbances being among the first and most common.

Symptoms of Selenium Toxicity

Diarrhea is often accompanied by other signs of selenosis, which can range from mild to severe depending on the dose and duration of excessive intake. These symptoms include:

Gastrointestinal Upset: Nausea, vomiting, and abdominal pain often occur alongside diarrhea.
Garlic Breath: A distinct garlic-like odor on the breath is a classic and unique symptom of selenium toxicity.
Changes to Hair and Nails: High selenium levels can cause hair loss (alopecia), and nail brittleness, discoloration, or loss.
Fatigue and Weakness: Generalized fatigue and unusual weakness are commonly reported symptoms.
Dermatitis: Some individuals may develop a skin rash or other skin-related issues.
Nervous System Issues: In severe cases, toxicity can lead to nervous system problems like tremors, tingling sensations (paresthesia), and irritability.

The Role of Dosage and Form

The risk of experiencing side effects like diarrhea is heavily dependent on both the dose and the chemical form of the selenium supplement. The inorganic forms, sodium selenite and sodium selenate, are less efficiently utilized and can be more toxic at lower doses than organic forms like selenomethionine and selenium-enriched yeast.

Comparison of Selenium Supplement Forms


Feature	Organic Selenium (e.g., Selenomethionine, Selenium Yeast)	Inorganic Selenium (e.g., Sodium Selenite, Sodium Selenate)
Bioavailability	High; body absorbs up to 90%.	Lower; body absorbs about 50% for selenite.
Toxicity	Relatively lower toxicity risk compared to inorganic forms.	Higher toxicity risk, especially in high doses.
Storage	Better stored in body tissues for later use.	Less readily stored by the body.
Gut Impact	Generally considered safer for the gut microbiome.	Can be more irritating to the gastrointestinal tract.

How Selenium Affects the Gut

While an essential nutrient, selenium's influence on the gut is complex and dependent on its concentration. Proper selenium levels support a healthy gut microbiome and protect the intestinal lining from oxidative damage. However, as intake moves towards toxic levels, it can trigger significant and harmful changes:

Disruption of the Microbiome: Both deficiency and excess selenium can disrupt the delicate balance of gut flora, leading to dysbiosis. High doses can negatively alter the composition of intestinal bacteria.
Inflammation: Excessive selenium can trigger inflammatory responses in the small intestine, increasing pro-inflammatory cytokines that damage intestinal tissue.
Impaired Intestinal Barrier: Studies have shown that selenium deficiency can lead to damage of the intestinal mucosal barrier by breaking down tight junction proteins that hold cells together. While this is a result of deficiency, it highlights the sensitivity of the intestinal environment to selenium concentration. A toxic excess can also exacerbate this issue.

Steps to Take If You Experience Diarrhea

Stop Supplementing: The first and most crucial step is to immediately stop taking the selenium supplement. This will halt further intake and prevent the toxicity from escalating.
Consult a Healthcare Provider: Speak with a doctor or pharmacist to discuss your symptoms. They can help determine if the supplement is the cause and can offer guidance on managing your symptoms and restoring balance.
Assess Your Dosage: Review the supplement's label and compare your intake to the established Tolerable Upper Intake Level (UL) of 400 mcg for adults. If your intake is consistently above this amount, toxicity is a plausible explanation for your symptoms.
Consider Other Sources: Evaluate your overall diet, especially if you consume Brazil nuts, which are exceptionally high in selenium. Combining high-dose supplements with a selenium-rich diet can easily push your intake into the toxic range.

Conclusion

While selenium is an essential mineral, the answer to "can selenium supplements cause diarrhea?" is a definitive yes, particularly in cases of excessive intake leading to toxicity, or selenosis. Diarrhea is one of the primary gastrointestinal symptoms of selenium poisoning, which is also characterized by a garlic-like breath odor, hair loss, and nail changes. To avoid this, it is crucial to stay within the recommended daily allowance (RDA) and not exceed the 400 mcg daily upper limit for adults unless under medical supervision. Always prioritize getting nutrients from a balanced diet and consult a healthcare professional before beginning any new supplement regimen. For more information on dietary supplements, you can consult resources like the Office of Dietary Supplements at the National Institutes of Health.

Frequently Asked Questions

No, diarrhea is not a common side effect when selenium is taken in recommended daily doses. It is, however, a hallmark symptom of selenium toxicity (selenosis), which occurs with excessive intake, typically from high-dose supplements.

For adults, the Tolerable Upper Intake Level (UL) is 400 micrograms (mcg) per day. Consistently taking doses above this limit is considered high and significantly increases the risk of toxicity, including diarrhea.

Initial symptoms often include gastrointestinal issues like nausea, vomiting, and diarrhea. A distinct garlic-like odor on the breath is another early and common indicator of selenium toxicity.

You should immediately stop taking the supplement. It is crucial to consult a healthcare provider to confirm the cause of your symptoms and receive guidance on managing them and restoring your mineral balance.

Yes, it does. Organic forms like selenomethionine are better absorbed and less toxic than inorganic forms such as sodium selenite. Supplements using inorganic forms may be more likely to cause gastrointestinal issues if taken in excess.

Interestingly, both excessive and deficient selenium levels can disrupt gut health. Studies have shown that selenium deficiency can lead to an imbalanced gut microbiome and inflammation, which may contribute to digestive problems like diarrhea.

You can obtain selenium safely from a balanced diet by including foods like Brazil nuts (in moderation), seafood, poultry, eggs, and whole grains. Dietary intake is generally safer and less likely to cause toxicity than supplements.