Yes, you can take magnesium and selenium together, at the same time or apart. We found no study that tested the pair and no documented absorption conflict between them. What needs watching is totals and units. The adult upper limit for supplemental magnesium is 350 mg (milligrams), counting supplements and medicines but not food; for selenium it is 400 mcg (micrograms), a unit a thousand times smaller. Both can turn up in multivitamins and thyroid or sleep blends, so add up every label. Selenium plays a role in thyroid hormone metabolism, but selenium trials in autoimmune thyroiditis have mixed results, and the American Thyroid Association recommends against selenium supplements for pregnant women with thyroid autoimmunity. Ask a doctor first if you have kidney disease or take thyroid medication, a bisphosphonate, a tetracycline or a quinolone antibiotic.
This content is for informational purposes only and is not a substitute for professional medical advice, diagnosis, or treatment. Always consult your physician or qualified healthcare provider before starting any supplement regimen.
Check your whole list, not just one pair
Supplement Tracker reminds you of each dose, and its interaction check looks at every supplement you take, with sources from the NIH.
A "thyroid support" capsule has joined your routine, and the back of the bottle reads "Selenium 200 mcg." The magnesium glycinate you take at bedtime reads "Magnesium 200 mg." Same number, one letter apart, and that letter is worth a factor of a thousand. So can you take magnesium and selenium together, or does one of them need its own time of day?
Yes, you can take them together. We found no study that tested the pair and no documented absorption conflict between them, so the same handful and separate times of day are both fine. What needs attention is arithmetic. Both minerals can turn up in more than one product, and their adult upper limits look almost identical on paper: 350 mg of supplemental magnesium, 400 mcg of selenium.
Do Magnesium and Selenium Interfere?
No known conflict. We found no study showing that either mineral lowers the other's absorption. That is an absence of evidence rather than proof that no effect exists, but it leaves no documented problem for a gap to solve.
The interactions worth planning around sit elsewhere. The magnesium fact sheet from the NIH Office of Dietary Supplements (ODS) flags some medicines: "Magnesium may interact with certain medications, such as oral bisphosphonates, tetracyclines, and quinolone antibiotics." And if iron is in the same routine, our iron and magnesium page covers whether that pair needs a gap.
What Each One Does
Magnesium. According to ODS, magnesium "plays a role in the active transport of calcium and potassium ions across cell membranes, a process that is important to nerve impulse conduction, muscle contraction, and normal heart rhythm." In severe cases, deficiency can cause hypocalcemia or hypokalemia (low blood calcium or potassium). Our guide to magnesium deficiency symptoms covers who is at risk and what a blood test can show.
For absorption, the form of magnesium matters; selenium, as far as we found, does not. ODS notes that "forms of magnesium that dissolve well in liquid have higher absorption than other forms," and that the aspartate, citrate, lactate and chloride forms tend to have higher bioavailability than magnesium oxide and magnesium sulfate. Glycinate is not part of that comparison; our glycinate versus oxide guide looks at where it fits.
Selenium and the thyroid. The body builds selenium into selenoproteins, which, according to the ODS selenium fact sheet, "play critical roles in thyroid hormone metabolism, DNA synthesis, reproduction, and protection from oxidative damage and infection."
A role in thyroid hormone metabolism does not by itself show that extra selenium helps a thyroid. In chronic autoimmune thyroiditis (Hashimoto's), trials that measured thyroid antibodies have given mixed results, according to ODS. The same fact sheet notes that "the American Thyroid Association recommends against selenium supplementation in pregnant women who have thyroid autoimmunity." If your thyroid blend also contains zinc, our selenium and zinc page covers that pair.
As a pair. We found no trial suggesting that magnesium and selenium do more together than apart, so each has to earn its place on its own evidence. Sharing a time slot is a convenience, not a strategy.
Practical Recommendations
This is our suggested routine, not a tested protocol.
- Choose the timing that fits your day. With no known conflict, there is no required gap. Taking selenium and magnesium together at bedtime is fine, and so is selenium with breakfast and magnesium at night. Our guide to the best time to take magnesium covers night, morning and split doses.
- Count every product. Selenium can come in a multivitamin and a thyroid blend, magnesium in a multivitamin, a sleep blend and a bottle of its own. Write down what each product provides, total each mineral separately, and redo the sum whenever a new bottle arrives. Our guide to auditing a crowded stack walks through the same exercise for a whole routine.
- Keep each mineral in its own unit. Add selenium in micrograms (printed as mcg or µg) and magnesium in milligrams, then hold each total against its own limit, never the other one's.
- Count magnesium from medicines. The magnesium limit covers medications as well as supplements, so if an antacid or laxative lists magnesium among its active ingredients, it belongs in the total.
Dosage Considerations
Magnesium: the magnesium upper limit for adults is 350 mg a day from supplements and medications, according to the ODS fact sheet; magnesium in food does not count toward it. That can look odd next to the recommended intake for adults, which ODS gives as 310 to 420 mg. The recommended intake counts magnesium from every source, food included, while the upper limit covers only supplements and medicines, so the two figures do not contradict each other. ODS also notes that "high doses of magnesium from dietary supplements or medications can cause diarrhea, nausea, and abdominal cramping." For children and adolescents, the limit runs from 65 to 350 mg, depending on age.
Selenium: the selenium upper limit is 400 mcg a day for adults, per the ODS selenium fact sheet. In milligrams that is 0.4 mg, which makes the gap plain: the magnesium limit is nearly 900 times larger.
How totals creep: start with the two bottles from the top of this page, 200 mcg of selenium in the thyroid blend and 200 mg of magnesium in the glycinate. Add a multivitamin with 100 mg of magnesium and 70 mcg of selenium, and a sleep blend with 100 mg of magnesium. Selenium comes to 270 mcg, well inside its limit. Magnesium comes to 400 mg: the same number as the selenium limit, and 50 mg over its own. A separate 200 mcg selenium capsule would take selenium to 470 mcg, over its limit too. No single label on that shelf looks large.
Who should check first:
- People with kidney disease. The kidneys regulate magnesium, so ask your doctor before taking it.
- People on certain prescription drugs. Magnesium may interact with oral bisphosphonates, tetracyclines and quinolone antibiotics, so ask your pharmacist how to fit it around them. Diuretics and proton pump inhibitors can affect magnesium status; if you take one, ask whether your level needs checking.
- Anyone who takes medication for thyroid disease. Selenium is involved in thyroid hormone metabolism and its trial record is mixed, so ask your doctor before adding it, and ask how your other supplements, magnesium included, should be timed around the medication.
- Pregnant women with thyroid autoimmunity. The American Thyroid Association recommends against selenium supplementation for them.
- Parents. The selenium limit above is an adult figure, so ask a pediatrician before giving either mineral to a child.
Key Takeaway
You can take magnesium and selenium together, in the same slot or apart; we found no conflict between them and no trial showing they do more as a pair. The work is in the totals: 350 mg of supplemental magnesium and 400 mcg of selenium a day for adults, counted across every label and each kept in its own unit. If you have kidney disease or thyroid disease, are pregnant with thyroid autoimmunity, or take a bisphosphonate, tetracycline or quinolone antibiotic, ask your doctor or pharmacist first.
Track Your Timing
Supplement Tracker groups your supplements into time-of-day slots, each with its own reminder time, so magnesium and selenium can share the bedtime slot or sit in separate ones. Log the multivitamin and the blends too and check off each dose as you take it; the dated history then shows what you actually took on any given day, ready for the next time a doctor asks.
