Is Magnesium Oxide a Waste of Money?
Ken D. Berry, MD, family physician
Published 11/9/26
Yes. The best-selling magnesium supplements are magnesium oxide, and research shows people absorb only about 4% of magnesium oxide. The rest you just poop out, so you are wasting about 96% of your money. A form your body can actually absorb, like magnesium citrate or magnesium malate, is the better buy.
Which magnesium supplements are magnesium oxide?
The three big brands are Nature's Bounty, Nature Made and Nature's Truth. They come in 250 milligram, 400 milligram and 500 milligram strengths, and the label tells you how much is in there. The problem is that all three are magnesium oxide.
How much magnesium oxide does your body absorb?
Only about 4%. A study measuring four commercial magnesium preparations found the fractional absorption of magnesium oxide was 4 per cent, with significantly higher absorption from magnesium chloride, magnesium lactate and magnesium aspartate. So when you take magnesium oxide, about 96% of what you paid for goes right through you.
Why do doctors prescribe magnesium oxide?
For the first few years of my medical practice, anytime someone had hypomagnesemia, which means low magnesium, I prescribed magnesium oxide. I did that because I didn't know better. That's what I was trained to prescribe by my mentors in medical school.
The only prescription magnesium in the United States is magnesium oxide, and even that is not FDA approved for low magnesium levels. It is only FDA approved for dyspepsia, which is indigestion. So every doctor who prescribes you magnesium oxide for low magnesium is doing so off label, which is legal and ethical. In my view, the reason it is so common is that it was the only magnesium you could write a prescription for that insurance would pay for.
What kind of magnesium should you take instead?
It depends on your bowels:
- If you tend toward constipation, take magnesium citrate. It helps loosen up your bowel movements.
- If you don't have any problem with constipation, take magnesium malate. There are several other absorbable forms too, and you can find them on Amazon.
What should you do with the magnesium oxide you already have?
Go get your bottle right now and read the ingredients. If it says magnesium oxide, you've been wasting most of your money. Go ahead and use it up. It's not bad for you, it's not going to hurt you, and it's not harmful. You're just wasting your money. When it runs out, buy a magnesium formulation your body can actually absorb.
Frequently asked questions
How much magnesium oxide do you absorb?
About 4%. A study of commercial magnesium preparations measured magnesium oxide's fractional absorption at 4 per cent.
Is magnesium oxide harmful?
No. It's not bad for you and it won't hurt you. You're just absorbing very little of it, so you're wasting your money.
Which magnesium is best for constipation?
Magnesium citrate, because it helps loosen up your bowel movements.
Which magnesium should I take if I'm not constipated?
Magnesium malate, or one of the several other absorbable forms you can find on Amazon.
Is prescription magnesium oxide FDA approved for low magnesium?
No. In the United States it is only FDA approved for dyspepsia, so prescribing it for low magnesium is off label, which is legal and ethical.
This information is not medical advice. Work with your own doctor on your health and medications.
Research referenced in the video
Magnesium absorption study (2001), Firoz M, Graber M, "Bioavailability of US commercial magnesium preparations": https://pubmed.ncbi.nlm.nih.gov/11794633/
About Dr. Berry
Dr. Ken Berry is a board-certified family physician who has cared for over 25,000 patients in more than two decades of practice in rural Tennessee. He grew up in Hohenwald, Tennessee, and earned his MD at the University of Tennessee Health Science Center.
He is the author of the #1 bestselling Lies My Doctor Told Me and the creator of the Proper Human Diet.
He lives on a farm in Holladay, Tennessee, with his wife Neisha and their two children.
