Selenium

Is selenium safe?

At the amounts most people get from food, yes. As a supplement, selenium needs more care than most minerals because the gap between what you need and what harms you is small. Adults need 55 mcg a day. The US upper limit is 400 mcg, and EFSA cut its limit to 255 mcg in 2023. A typical American diet already supplies around 100 mcg, from bread, meat, fish and eggs.

The usual supplement dose of 100 to 200 mcg therefore puts many people at two to five times their needs. That is not toxic in itself, but there is little room for a second product, a handful of Brazil nuts or a manufacturing error.

The 2008 selenium poisoning outbreak

The clearest example of selenium's narrow margin came from a product, not a dose choice. In March 2008, after reports from Florida and Tennessee, the FDA warned consumers not to use Total Body Formula (Tropical Orange and Peach Nectar flavours) and Total Body Mega Formula (Orange/Tangerine). The Atlanta distributor, Total Body Essential Nutrition, recalled them.

The investigation, published in the Archives of Internal Medicine in 2010, found that the liquid supplement contained 200 times the selenium on its label. It identified 201 cases in 10 states. The median dose people took was about 41,700 mcg a day, roughly 760 times the RDA. The commonest symptoms were diarrhoea (78%), fatigue (75%), hair loss (72%), joint pain (70%), nail problems (61%) and nausea (58%). One person was hospitalised. Ninety days later, about half still had discoloured or lost fingernails, and 29% still had hair loss.

The authors concluded that pharmaceutical manufacturing standards might have prevented it. It was not the first such case: the Linus Pauling Institute records 13 people poisoned by tablets that, because of a manufacturing error, contained 27.3 mg (27,300 mcg) each. This history is why selenium scores 70 on quality while simpler minerals such as zinc score higher.

Chronic excess: selenosis

Long-term overdose builds up more slowly. The signs of selenosis are brittle hair and nails that may fall out, a garlic smell on the breath, skin rashes, fatigue, irritability and, in more severe cases, nerve damage. In a region of China with very high selenium in the soil, toxic effects became more frequent at intakes of about 850 mcg a day. The US upper limit of 400 mcg was set to prevent the earliest signs, hair and nail changes.

Brazil nuts are the easiest way to overshoot. The Linus Pauling Institute lists about 544 mcg in a one-ounce serving of six nuts, though the content depends heavily on the soil. Eating them daily on top of a selenium pill is the most common route to excess.

Selenium and diabetes

The other concern is subtler. In the Nutritional Prevention of Cancer trial, 200 mcg of selenium a day for an average of 7.7 years raised the risk of type 2 diabetes in people who already had the highest selenium levels at the start. In the larger SELECT trial of 35,533 men, the selenium group had slightly more diabetes (relative risk 1.07), though the difference was not statistically significant. Other trials found no effect. The Linus Pauling Institute advises people with high selenium status or at risk of type 2 diabetes to avoid selenium supplements.

SELECT also found that selenium did not prevent prostate or any other cancer, which was the main reason many people had taken it.

Interactions

Selenium has few known drug interactions, its strongest factor. The anticonvulsant valproic acid and the cancer drug cisplatin can lower selenium levels. No important interactions with common drugs such as warfarin or statins are established. Selenium is often combined with vitamin A and other antioxidants in "immune" or "eye" formulas, so check every label for overlapping doses.

How to stay within safe limits

• Add up selenium from every product, including multivitamins.
• Count Brazil nuts as a supplement.
• Choose a USP VerifiedThird-party testingTesting of a supplement by an independent lab to check that it contains what the label says and is free of set contaminants.More in the glossary or NSF-certified product; liquids and very cheap imports carry more risk of dose errors.
• Stop and see a doctor if your hair starts falling out, nails change or your breath smells of garlic.
• If you have type 2 diabetes or prediabetes, or a thyroid condition, talk to your doctor before taking selenium.

How we scored it

Selenium scores 74/100. Interactions (88) are its strongest factor. Dose safety (68) and quality (70) are the weakest: the margin between need and harm is narrow, and two documented manufacturing errors poisoned more than 200 people. Organ toxicity (72) and regulatory actions (76) reflect selenosis, the diabetes signal and the 2008 FDA warning.