Does selenium have side effects?

It’s completely normal to have a few questions when you start taking a new supplement. After all, you want to support your health, not risk making it worse! Learning more about the possible side effects associated with a supplement isn’t being paranoid – it’s smart.

The good news is that selenium is an essential mineral for our bodies. Because of this, most people get on with it very well. However, just like with any change to your diet or routine, there are a few things you should consider.

In this guide, we walk you through the potential side effects of selenium, from rare allergic reactions to common signs you might be taking a little too much.

Can you be allergic to selenium?

It might seem odd, but it is possible to be allergic to certain supplements – though fortunately, it’s rare.

A true selenium allergy is very uncommon. However, supplements aren’t just made of pure selenium. They often contain other ingredients to hold the tablet together or form the capsule shell, and these ingredients might trigger allergic reactions for some people.

What to look out for

If you do have an allergic reaction, your body will usually let you know quite quickly. It’s helpful to know the signs so you can act fast just in case. These are:

Skin Reactions: This is often the first sign. You might notice a rash appearing, your skin feeling itchy or hives (raised red bumps) appearing on your body.¹
Swelling: Some people experience swelling, particularly around the face, lips, tongue, or throat.
Trouble Breathing: In more serious cases, you might feel wheezy or struggle to catch your breath.
Dizziness: Feeling suddenly lightheaded or very weak.

What should you do?

If you notice any of these symptoms after taking a supplement, you should stop taking it immediately.

If the symptoms are severe, especially if you have trouble breathing or your throat feels tight, this is a medical emergency. Seek urgent medical attention by calling 999.

Again, please remember that this type of reaction is rare. The majority of people take selenium without any allergic issues at all.

What are the side effects of selenium supplements?

If you’re not allergic, what else should you expect? For the vast majority of people, the answer is “not much.”

Selenium is generally very well received when taken at the recommended dose.² Most people can simply add it to their daily routine and get on with their day, noticing nothing but the benefits over time.

However, our bodies are all unique. Occasionally, the introduction of a new supplement may lead to some challenges as your system adjusts.

The “Settling In” Period

When you first start taking selenium, you might experience some mild tummy trouble. This is quite common with many mineral supplements, not just selenium.

You might feel a bit of nausea or have some mild stomach cramps. Some people also find their toilet habits change slightly, perhaps experiencing a bit of diarrhoea for the first few days.

These tummy issues are often caused by taking supplements on an empty stomach. Taking selenium with meals may ease digestion, as food helps to cushion your stomach and often helps your body to absorb nutrients better.

Signs you might be taking too much

While selenium is essential for health, the body only needs a tiny amount. That’s why it’s called a “trace” mineral. Because we need so little, there is a limit to how much we should take.

If you take high doses of selenium over a long period, it can build up in your body and cause problems. This condition is sometimes called selenosis.³ Fortunately, it’s very unlikely you’ll experience selenosis from your diet alone.

However, you may get selenium toxicity symptoms if you accidentally take too many supplements or combine supplements with lots of selenium-rich foods (like eating a whole bag of Brazil nuts every day).

Here are the signs that your selenium levels might be getting too high:

Hair and Nail Changes

One of the most classic signs of excess selenium affects your beauty regime. You might notice your hair becoming dry and brittle.¹ In some cases, people notice more hair falling out than usual when brushing or washing.

Similarly, your nails might become weak or start to crack and peel easily. If you notice your nails looking white, it might be a sign to check your dosage.²

Garlic Breath

This symptom is a strange one, but it’s a very common sign of high selenium levels. Excess selenium in your system can give your breath a distinct metallic or garlic-like smell.¹

If you haven’t been eating garlic bread but friends are offering you mints, it might be worth reviewing your supplements.

Feeling Tired and Irritable

We often take supplements to boost our energy, but taking too much selenium can actually do the opposite. You might feel unusually fatigued or weak.³

Alongside the tiredness, some people feel irritable or just not quite themselves.¹ This can be hard to notice since we all have tired days, but if it continues, note it down.

Ongoing Tummy Trouble

We mentioned that mild stomach issues can happen when you start supplements. If you’ve been taking selenium for a while and still feel sick, vomit or have an upset stomach, your dose may be too high.³

Metallic Taste

Similar to the garlic breath, some people report a constant metallic taste in their mouth that doesn’t go away with brushing their teeth.¹

Staying Safe

Reading this list might make selenium sound a bit scary, but it really shouldn’t be. These side effects are almost exclusively linked to taking too much.

To stay safe, simply follow these golden rules:

Stick to the recommended dose: Don’t be tempted to take double the amount thinking it will do double the good. The packet instructions are there for a reason.
Check your other supplements: Sometimes, a multivitamin might already contain selenium. If you take that plus a separate selenium tablet, you might be getting more than you realise.
Listen to your body: If you notice brittle nails or that garlic breath we mentioned, pause the supplement and speak to a medical professional.

When to speak to a doctor

If you have any underlying health conditions or if you’re taking other medications, have a quick chat with your GP or a pharmacist before starting selenium. They can give you peace of mind that it’s safe to take alongside your other meds – or recommend alternatives if it isn’t.

In summary, selenium is a safe and helpful mineral for most of us. By sticking to the guidelines and listening to your body, you can enjoy the benefits without the worry.

References

Selenium: Fact Sheet for Health Professionals. National Institutes of Health (NIH), Office of Dietary Supplements.
Selenium: Vitamins and Minerals. National Health Service (NHS) UK.
Selenosis: Potential adverse effects of high intake. Shreenath AP, Hashmi MF, Dooley J. Selenium Deficiency. [Updated 2023 Oct 29]