The short answer
Magnesium oxide is 60% magnesium, so tablets are small and cheap, but only a small fraction is absorbed: about 4% in one study. Much of the rest stays in the gut and draws in water, which makes it an effective laxative. It is a poor choice for raising magnesium levels and a good one for constipation.
On this page
How much magnesium does it contain?
Magnesium oxide (MgO) is 60% magnesium by weight, far more than any other form. A 500 mg tablet holds about 300 mg of magnesium, enough for a full day in one small tablet.
How well is it absorbed?
Poorly. A 2001 study measured fractional absorption of magnesium oxide at about 4%, much lower than magnesium chloride, lactate or aspartate. In a 2003 University of Reading trial, people taking 300 mg of magnesium a day as oxide for 60 days ended up no better off than those on placebo, while citrate raised their levels.
Oxide doesn’t dissolve well in water and relies on stomach acid to break down, which may be one reason. The forms comparison puts these figures side by side. Absorption may be even lower in people taking acid-reducing medicines.
What is magnesium oxide good for?
Constipation. The magnesium that isn’t absorbed stays in the gut and holds water, softening stools. In a 2021 trial, 1.5 g of magnesium oxide a day for four weeks improved symptoms in 68% of people with chronic constipation, compared with 12% on placebo. See constipation.
Indigestion. Magnesium oxide and hydroxide neutralise stomach acid and are found in some antacids. Count these if you also take a supplement: see side effects.
What has it been tested for, and failed?
Several of the most-cited negative trials used oxide:
- Night-time leg cramps: in a 2017 trial of 94 adults, magnesium oxide was no better than placebo. See muscle cramps.
- Menopausal hot flushes: in a 2015 trial of 289 women, 800 or 1,200 mg of magnesium oxide a day made no difference, though diarrhoea was more common.
It is fair to ask whether better-absorbed forms would have done better. No trial has tested that directly.
Why is it in so many cheap supplements?
Because it packs the most magnesium into the smallest, cheapest tablet, so a label can claim a big number. Per milligram absorbed, it is often poor value. Our buying guide shows how to compare products by cost per 100 mg, and citrate or glycinate are the usual alternatives if you want to raise your levels.
Who should be careful with magnesium oxide?
Anyone with kidney disease, and older people using it every day as a laxative. Very high blood magnesium is rare but can be fatal: a 2018 case report described a death from long-term magnesium laxative use in a woman with normal kidney function. Oxide also binds to several medicines in the gut. See medicines it affects.
Keep reading
- Magnesium L-threonate: does the brain magnesium live up to the hype?
Magnesium L-threonate (Magtein) is sold for memory and sleep. What the rat study and industry-linked human trials show, and whether it's worth it.
- Epsom salt baths and magnesium sprays: can skin absorb magnesium?
Do Epsom salt baths, magnesium flakes, oils and sprays raise your magnesium? What the research says about absorption through the skin.
- Magnesium glycinate: benefits, dose and how it compares
What magnesium glycinate (bisglycinate) is, how much magnesium a capsule really holds, the trial evidence for sleep, and how it compares with citrate.
Common questions
Is magnesium oxide a waste of money?
Not if you want a laxative, where it works well. For topping up magnesium it is less efficient than citrate or glycinate, but because each tablet holds so much magnesium, it still delivers some, and many trials of magnesium for blood pressure, sleep and migraine in children used oxide.
Why do so many supplements use magnesium oxide?
Because it packs the most magnesium into the smallest, cheapest tablet, so the front of the pack can show a big number. Check the absorption, not just the milligrams.
Can I take magnesium oxide every day?
At normal supplement doses, most healthy adults can. If you use it daily as a laxative, talk to your GP, because magnesium can build up in the blood, especially in older people and anyone with reduced kidney function.
Is magnesium oxide good for sleep?
It's not the best choice. It's poorly absorbed and more likely to loosen stools. The best sleep trial used bisglycinate, and gentler forms suit most people better.
References
- Firoz M, Graber M. Bioavailability of US commercial magnesium preparations. Magnesium Research, 2001.pubmed.ncbi.nlm.nih.gov
- Walker AF et al. Mg citrate found more bioavailable than other Mg preparations in a randomised, double-blind study. Magnesium Research, 2003.pubmed.ncbi.nlm.nih.gov
- Morishita D et al. Senna versus magnesium oxide for the treatment of chronic constipation: a randomized, placebo-controlled trial. American Journal of Gastroenterology, 2021.pubmed.ncbi.nlm.nih.gov
- Roguin Maor N et al. Effect of magnesium oxide supplementation on nocturnal leg cramps: a randomized clinical trial. JAMA Internal Medicine, 2017.pubmed.ncbi.nlm.nih.gov
- Park H et al. A double-blind placebo-controlled study of magnesium supplements to reduce menopausal hot flashes (NCCTG N10C2). Menopause, 2015.pubmed.ncbi.nlm.nih.gov
- Bokhari SR et al. Fatal hypermagnesemia due to laxative use. American Journal of the Medical Sciences, 2018.pubmed.ncbi.nlm.nih.gov
First published 4 November 2026. Spotted an error? Tell us and we'll correct it. Read our editorial policy.