If you’ve been staring at two magnesium tubs trying to work out why one costs £4 and the other costs £18, this is the comparison you actually need. I’m a registered Nutritionist, and magnesium oxide vs glycinate is one of the most lopsided supplement debates out there. The answer is clearer than the marketing makes it look.

Let’s get into it.

The Short Answer

For daily use, magnesium glycinate. For a short-term bowel-moving effect, magnesium oxide.

That’s the headline.

The longer version: glycinate is generally better absorbed and much kinder on your gut, so it’s the form most people actually want when they say they want a magnesium supplement [3][4]. Oxide has notoriously poor absorption, around 4% in one widely cited human study [1], which is exactly why it works as a laxative and exactly why it’s a poor choice for daily use.

Quick Comparison

Why Magnesium Oxide Is Everywhere and Why That’s a Problem

Short answer: because it’s cheap and looks good on a label.

Magnesium oxide is made by burning magnesium with oxygen, and it contains roughly 60% elemental magnesium by weight. That’s a big number, and big numbers sell. It’s why you’ll see “400mg magnesium!” on the front of a £3 supermarket tub and wonder why anyone would pay more.

Here’s the catch. High elemental content and high absorption are not the same thing. Magnesium oxide is poorly soluble, and human studies consistently find it’s poorly absorbed compared with better-soluble organic or chelated forms [1][2][4].

So what happens to the rest of it? It sits in the gut and pulls water into the bowel. That’s the same osmotic effect that makes magnesium salts work as laxatives [11].

To be fair to oxide, that’s a legitimate use. If a short-term bowel-moving effect is what you want, oxide can do that job cheaply. What it can’t do well is steadily improve your magnesium status over time [1][4].

Why Magnesium Glycinate Is the Form Worth Paying For

What About Sleep?

This is the main reason people come to glycinate in the first place.

The broader magnesium-and-sleep literature is promising but mixed, not miraculous [6].

The glycinate-specific picture got a bit stronger in 2025 with a randomised placebo-controlled trial showing modest improvements in insomnia severity [7].

In practice, if you sleep poorly and your diet is low in magnesium, glycinate is a sensible form to try. Just don’t expect it to work like a sedative!

Magnesium Oxide vs Glycinate: The Key Differences

Which One Actually Gets Absorbed?

In one often cited study study magnesium oxide had an absorption of about 4% [1]. Not great!

Earlier human work also found oxide less bioavailable than magnesium citrate [2] and systematic reviews broadly conclude that organic forms outperform inorganic ones on bioavailability [4]. For glycinate specifically, one study found magnesium diglycinate was better absorbed and better tolerated than oxide [3].

One thing that often gets missed in this conversation: Fractional absorption actually drops as the dose gets bigger [5]. So a huge number on the label is not the same as a huge amount getting into your body. This is also why I’d steer people away from mega-dose magnesium products without medical supervision.

The Gut Tolerance Gap

Ever taken a magnesium supplement and felt your stomach start staging a protest? That’s usually unabsorbed magnesium drawing water into the bowel.

With oxide, that’s a feature if you want that effect but a nightmare if you don’t! [11].

Magnesium glycinate is generally the gentler option. In practical terms, it’s the form you could arguably choose, based on the emerging evidence, for someone with a sensitive gut, someone who wants daily use, or someone who’s been scared off magnesium once by a cheap oxide tablet.

The Cost Trap Most People Fall Into

Magnesium oxide is cheap. Sometimes dramatically cheap. Magnesium glycinate costs more, and that’s the part people understandably notice first.

But cost per tablet is the wrong question.

Cost per useful dose is the better one. If a form is absorbed poorly and gives you digestive grief, the bargain starts looking a lot less clever. That’s why oxide can make sense as a cheap laxative-style option, but not usually as the smart buy for day-to-day magnesium support.

Use Cases: Where Each Form Actually Fits

Magnesium oxide fits one main use case: you specifically want the osmotic bowel effect that comes with poor absorption [11]. Beyond that, it is not the form I’d choose first for replenishing magnesium intake, ongoing daily use, or trying to find a gentler supplement based on the evidence.

Magnesium glycinate fits most day-to-day use cases much better.

Evidence is starting to suggest it’s the form to reach for when someone wants a magnesium supplement that’s more likely to be absorbed and less likely to upset their stomach. For sleep, the evidence is encouraging but mixed overall, so I prefer saying it may support sleep quality rather than pretending it’s a magic switch [6][7].

The same applies to stress and anxiety. There are promising signals, but the overall evidence quality is not especially strong, so this is a “may help some people” area rather than a slam dunk [8].

For a deeper dive into how glycinate compares to another popular form, my article on magnesium glycinate vs citrate covers that ground thoroughly.

Full Comparison Table

Which Should You Choose?

Before reaching for any supplement, it’s worth remembering that magnesium is abundant in food. Pumpkin seeds are one of the richest sources, with a small handful making a decent contribution to your intake. Dark chocolate, spinach, almonds, legumes, and wholegrains are all solid dietary sources. Food first, always.

That said, if you’re supplementing, here’s how to think about it:

If you want to support sleep quality: magnesium glycinate is the form I’d lean towards. Evening use is reasonable if it suits you, but consistency and tolerance matter more than obsessing over the exact minute you take it [6][7].

If you’re managing stress or anxiety: magnesium glycinate is a reasonable form to try, but keep expectations realistic. The evidence is suggestive rather than definitive [8].

If you want to support muscle function and you’re not getting much magnesium in your diet: glycinate is the more practical daily form. I just wouldn’t market magnesium as a guaranteed cramp fix [9][10].

If what you really want is that short-term bowel-moving effect: magnesium oxide is the one more likely to do that [11].

If you’ve grabbed a cheap supermarket magnesium thinking you’re covered: check the label. If it’s oxide, it may still have a place, but it’s probably not the form I’d choose for daily magnesium support.

For daily supplementation, I’d point you towards Ethical Nutrition’s Magnesium Glycinate. It’s the form I’d actually recommend rather than one I’m just noting exists. Use code ALEXSTEWART for 15% off, or it applies automatically through that link.

If you’re weighing up other magnesium forms, my piece on magnesium taurate vs glycinate is worth a read, particularly if you want a broader sense of how the different forms stack up.

Frequently Asked Questions

Is magnesium oxide useless as a supplement?

Not useless, just a poor fit for most daily magnesium goals. It has a clear place when the laxative-style effect is the reason you’re taking it, but for improving magnesium status over time, oxide is one of the least convincing options because of its poor absorption [1][2][11].

Why do so many supplements use magnesium oxide if it’s poorly absorbed?

Because it’s cheap to produce and lets manufacturers print a large magnesium number on the label. That’s commercially convenient, even if it’s not especially consumer-friendly.

Can magnesium glycinate cause loose stools?

Yes, especially at higher doses, but evidence suggests it’s generally much less likely to do that than oxide. That’s one of the main practical reasons people prefer it [4][11].

How much magnesium glycinate should I take per day?

Check the label for elemental magnesium, not just the total compound weight.

A sensible starting point is often around 100 to 200mg elemental magnesium per day, especially if your diet already contains some magnesium. In the UK, adult RNIs are 300mg/day for men and 270mg/day for women in total, and the NHS notes that 400mg/day or less from supplements is unlikely to cause harm [12].

If you have kidney disease, you’re taking medicines such as certain antibiotics, thyroid medication, and  just in general, it’s worth checking with your pharmacist or GP before adding a magnesium supplement, because timing and interactions can matter [13].

Key Takeaways

• Magnesium oxide is cheap and high in elemental magnesium on paper, but it is poorly absorbed in practice [1][2].
• Magnesium glycinate is a more sensible choice for day-to-day use because it is generally better tolerated and has a stronger evidence-based case than oxide for routine supplementation [3][4].
• The evidence for magnesium and sleep is promising but mixed, so “may support” is the honest language here, not miracle claims [6][7].
• Magnesium is not a guaranteed fix for muscle cramps, especially if deficiency is not part of the picture [9].
• Food sources still come first. Supplements are there to fill gaps, not replace a decent diet.

References

1. Firoz M, Graber M. Bioavailability of US commercial magnesium preparations. Magnes Res. 2001. PubMed
2. Lindberg JS, Zobitz MM, Poindexter JR, Pak CYC. Magnesium bioavailability from magnesium citrate and magnesium oxide. J Am Coll Nutr. 1990. DOI
3. Schuette SA, Lashner BA, Janghorbani M. Bioavailability of Magnesium Diglycinate vs Magnesium Oxide in Patients with Ileal Resection. JPEN J Parenter Enteral Nutr. 1994. DOI
4. Pardo MR, Garicano Vilar E, San Mauro Martín I, Camina Martín MA. Bioavailability of magnesium food supplements: A systematic review. Nutrition. 2021. DOI
5. Schuchardt JP, Hahn A. Intestinal Absorption and Factors Influencing Bioavailability of Magnesium-An Update. Curr Nutr Food Sci. 2017. DOI
6. Arab A, Rafie N, Amani R, Shirani F. The Role of Magnesium in Sleep Health: a Systematic Review of Available Literature. Biol Trace Elem Res. 2023. DOI
7. Schuster J, Cycelskij I, Lopresti A, Hahn A. Magnesium Bisglycinate Supplementation in Healthy Adults Reporting Poor Sleep: A Randomized, Placebo-Controlled Trial. Nature and Science of Sleep. 2025. DOI
8. Boyle NB, Lawton C, Dye L. The Effects of Magnesium Supplementation on Subjective Anxiety and Stress-A Systematic Review. Nutrients. 2017. DOI
9. Garrison SR, Allan GM, Sekhon RK, Musini VM, Khan KM. Magnesium for skeletal muscle cramps. Cochrane Database Syst Rev. 2020. PubMed
10. Wang R, Chen C, Liu W, et al. The effect of magnesium supplementation on muscle fitness: a meta-analysis and systematic review. Magnesium Research. 2017. PubMed
11. Mori H, Tack J, Suzuki H. Magnesium Oxide in Constipation. Nutrients. 2021. DOI
12. NHS. Others: vitamins and minerals. NHS website. NHS
13. National Institutes of Health Office of Dietary Supplements. Magnesium: Health Professional Fact Sheet. ODS