Yes, you can take selenium and magnesium together. These two minerals don’t compete for absorption, and research suggests they may actually work better as a pair than either one alone. There are no known adverse interactions between them at standard supplement doses.
Why These Two Minerals Work Well Together
Selenium and magnesium support several of the same body systems, and their biological roles overlap in ways that make co-supplementation logical. One of the clearest connections is in thyroid function. Magnesium is essential for converting inactive thyroid hormone (T4) into its active form (T3), and it also helps the thyroid use iodine properly. Selenium, meanwhile, protects thyroid tissue from oxidative damage during hormone production. A magnesium deficiency can actually reduce your selenium levels by changing how selenium is distributed and used in your tissues. So being low in one mineral can undermine the other.
Animal research published in Lipids in Health and Disease found that combined selenium and magnesium supplementation significantly lowered total cholesterol, LDL cholesterol, and triglycerides in subjects fed a high-fat diet. The combination also boosted antioxidant enzyme activity and reduced markers of oxidative stress and liver damage. Interestingly, the lower doses of both minerals trended toward stronger cholesterol-lowering effects than higher doses, suggesting more isn’t necessarily better.
No Known Absorption Conflict
Some minerals do interfere with each other’s absorption. Calcium and iron, for example, compete for the same transport pathways in the gut, so taking them together can reduce how much of each you absorb. Selenium and magnesium don’t share this problem. Selenium is absorbed primarily as an amino acid-like compound (selenomethionine or selenocysteine), while magnesium uses different ion channels. You can take them at the same time without worrying about one blocking the other.
How Much Is Safe
The recommended daily intake for selenium is 55 mcg for most adults, rising to 60 mcg during pregnancy and 70 mcg while breastfeeding. The tolerable upper limit, meaning the maximum daily amount unlikely to cause harm, is 400 mcg. Most selenium supplements contain between 50 and 200 mcg per dose.
For magnesium, the RDA is 310 to 320 mg for adult women and 400 to 420 mg for adult men, depending on age. The upper limit for supplemental magnesium (not counting what you get from food) is 350 mg. This distinction matters: the upper limit applies only to supplements and fortified foods, not to magnesium naturally present in your diet.
Staying within these ranges is straightforward with standard supplement products. Problems arise mainly from mega-dosing or combining multiple supplements that contain the same nutrient.
Signs You’re Taking Too Much
Selenium toxicity is rare at typical supplement doses but becomes a concern above 400 mcg per day over time. Early signs include a garlic-like taste in the mouth, brittle nails, hair loss, and skin rashes. Chronic excess can cause more serious neurological symptoms.
Magnesium toxicity from supplements most commonly shows up as diarrhea, nausea, and muscle weakness. These are the body’s early warning signs. At extremely high levels (far beyond what oral supplements typically deliver), magnesium can cause dangerous drops in blood pressure, loss of reflexes, and heart rhythm problems. People with kidney disease are at higher risk because the kidneys are responsible for clearing excess magnesium.
When and How to Take Them
Take magnesium with food. Research shows it’s better absorbed when paired with a meal, and taking it on an empty stomach increases the chance of nausea or diarrhea. Since selenium doesn’t have strong food-timing requirements, the simplest approach is to take both with the same meal.
If you’re taking magnesium primarily for sleep, taking it 30 to 60 minutes before bed works well, particularly with magnesium glycinate. In that case, you could take your selenium with breakfast or lunch and your magnesium later in the evening. But the exact timing matters less than consistency. Taking both at the same time every day keeps your levels stable, and that’s what produces the most reliable benefits.
Choosing the Right Forms
Magnesium comes in many forms, and they’re not all equally useful. Magnesium glycinate is well absorbed and gentle on the stomach, making it a popular choice for general supplementation and sleep support. Magnesium citrate absorbs well too but has a stronger laxative effect, which some people find helpful and others find inconvenient. Magnesium oxide is cheap but poorly absorbed, so you get less actual magnesium per pill.
For selenium, selenomethionine is the most common and well-absorbed form. Sodium selenite is another option but is slightly less bioavailable. Either form works fine alongside any form of magnesium. No specific magnesium-selenium form combination has been shown to cause problems or offer a clear advantage over another.