Supplements and safety
Can You Take Too Much Selenium?
Reviewed by a qualified clinician · analysed at UKAS-accredited UK labs (ISO 15189)
Last reviewed October 20265 min read
Every Helvy guide is written by our health editors, then checked by a qualified clinician before it goes live and re-checked as the science moves. We name clinical roles, not individuals, until each reviewer has agreed to be credited publicly. This is wellness guidance to help you understand your own data, not a diagnosis.
QUICK ANSWER
Yes. The NHS says 350 micrograms or less a day from supplements is unlikely to harm you, and the EU food safety body set a lower adult ceiling of 255. Early signs of too much are garlic breath, a metallic taste, brittle nails and hair shedding. Count every source, and check your thyroid before you take it.
Taking it for your thyroid? Build your test →As of October 2026. Selenium stacks for Hashimoto's are all over the supplement forums, and so is the worry that follows: my nails have gone brittle and my breath smells odd. Is it the selenium? It might be. The safe range is narrower than for most minerals, and in January 2023 the European Food Safety Authority lowered the ceiling most people had been working to.
1. What are the signs of too much selenium?
Your breath usually tells you first. The Vanderbilt poison control centre puts it plainly: “Early indicators of excess intake are a garlic odor in the breath and a metallic taste in the mouth.”
Keep going and the signs move to your hair, nails and skin. The condition is called selenosis, which simply means selenium overload. It builds over weeks of high intake. One large capsule will not cause it.
| Where you notice it | What it looks like |
|---|---|
| Breath and mouth | A garlic smell and a metallic taste. |
| Nails | Brittle, ridged nails that may lift or fall off. |
| Hair | Brittle hair that breaks at the scalp, then sheds. |
| Skin and gut | Rashes, nausea and diarrhoea. |
| Nerves and mood | Tiredness, irritability, and tingling or numbness. |
None of these signs is unique to selenium. Hair shedding and tiredness have commoner causes, such as low iron or a thyroid that is running slow. A full blood count shows anaemia, a shortage of red blood cells, which is one of the first things worth ruling out.
2. How much selenium is too much?
You need very little. The NHS puts the daily need at 75 micrograms for men and 60 for women aged 19 to 64. A microgram is a thousandth of a milligram, so this is a tiny amount. Most people get it from fish, eggs, meat and nuts.
“Taking 350µg or less a day of selenium supplements is unlikely to cause any harm.”
NHS, Vitamins and minerals
That is the UK line. Europe now draws a stricter one. In 2023 the European Food Safety Authority set an adult upper limit of 255 micrograms a day, from all sources combined. It picked hair loss as the earliest harm. It saw that harm at about 330 micrograms a day in a large US trial, then built in a safety margin.
So the two numbers measure different things. The NHS figure covers supplements alone. The EU figure covers food plus supplements. If you want one rule, keep your total under 255.
3. How do people end up taking too much?
Almost always by stacking. A standalone selenium capsule is commonly 200 micrograms. That alone sits under both limits. The trouble starts with what goes on top.
Say your multivitamin adds 55 micrograms. You are already at 255 before you count a single meal. A thyroid support blend often carries selenium too. Brazil nuts are the wild card. Harvard's nutrition team notes that a single nut can hold more than a day's need, and the amount swings with the soil the tree grew in.
Read the back of every label and add the selenium lines together. Our guide to selenium supplements in the UK covers who is genuinely short of it, and whether you need a multivitamin at all.
4. Taking it for your thyroid? Check that first
Most people on a high dose are taking it for their thyroid. The thyroid uses selenium to make its hormones work, so the idea makes sense. The evidence is thinner than the forums suggest.
A Cochrane review found the evidence for selenium in Hashimoto's, the autoimmune form of an underactive thyroid, “to support or refute the efficacy of selenium supplementation ... is incomplete.” Our guide to supplements for Hashimoto's sets out what the newer trials add.
So start with a number. TSH, thyroid stimulating hormone, is the signal your brain sends to push the thyroid harder. A high TSH means the thyroid needs pushing. Free T4 is the main hormone the thyroid actually makes. Read together, they show whether your thyroid levels are normal.
Helvy's Thyroid & Vital Organs panel (£159) reads TSH, free T4 and free T3, the active form, from one home finger-prick sample. The General Energy & Wellness panel (£149) reads TSH and free T4 alongside vitamin D, B12 and cortisol. Neither measures thyroid antibodies, which is a separate test to take with your GP.
Neither panel measures selenium itself, either. If you are worried about overload, a blood selenium level is a test to ask your GP about. The thyroid hub gathers the rest of our thyroid guides in one place.
5. What should you do if you think you took too much?
Stop the selenium supplement first. Then add up every other product you take that lists it. Mild signs like garlic breath usually ease once your intake falls.
Some signs need a clinician. Speak to your GP or call NHS 111 if you have numbness, tingling, a spreading rash, or hair and nail changes that do not settle. Take your supplement bottles with you, so they can see the doses. The same counting habit applies to zinc, which has its own UK limit.
READY TO TEST?
Know your thyroid numbers before you dose
Helvy's home blood tests read your TSH and free T4, so you can see where your thyroid stands before you take anything for it. Results in around five working days from UKAS-accredited UK labs.
Frequently asked questions
Can you take too much selenium?
Yes. The NHS says 350 micrograms or less a day from supplements is unlikely to cause harm, and the European Food Safety Authority set a lower adult ceiling of 255 micrograms a day in 2023. A 200 microgram capsule plus a multivitamin and a few Brazil nuts can pass both.
What are the first signs of too much selenium?
A garlic smell on the breath and a metallic taste are the early signs. With longer exposure, nails turn brittle or ridged and hair starts to shed. Nausea, skin rashes, tiredness and irritability are also reported. These come from weeks of high intake, not from one capsule.
Is 200mcg of selenium a day too much?
On its own, 200 micrograms sits under both the NHS 350 microgram line and the EU 255 microgram ceiling. The risk is what you add on top. Count your multivitamin, any thyroid blend and Brazil nuts, because those can push the total past 255.
Should I take selenium for my thyroid?
Check your thyroid first. A Cochrane review found the evidence for selenium in Hashimoto's thyroiditis incomplete. A TSH and free T4 result tells you where your thyroid actually stands, and a qualified clinician can help you decide whether a supplement makes sense.
What should I do if I think I've taken too much selenium?
Stop the supplement and add up everything else you take that contains selenium. Mild signs usually ease once intake falls. Speak to your GP or call NHS 111 if you have numbness, tingling, a rash or hair loss that does not settle. This guide is general information, not medical advice.
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