These two forms of magnesium are both sold as supplements, often at similar price points. But they work very differently — and for most people trying to correct a deficiency or support their health, the difference matters significantly.
Here’s a plain-English breakdown of what sets them apart.
Quick Comparison
| Magnesium Oxide | Magnesium Glycinate | |
|---|---|---|
| Bound to | Oxygen | Glycine (amino acid) |
| Absorption | Very low (~4%) | Substantially higher |
| Elemental Mg per gram | High (~60%) | Lower (~11%) |
| GI tolerance | Poor — laxative effect | Excellent |
| Best use | Constipation, heartburn, antacid | Daily supplementation, sleep, anxiety |
| Cost | Low | Moderate |
| Recommended for raising Mg levels | No | Yes |
What Is Magnesium Oxide?
Magnesium oxide is magnesium bound to oxygen. It’s one of the most common forms found in budget supplements and over-the-counter antacids, largely because it’s cheap to produce and contains a high percentage of elemental magnesium by weight (around 60%).
The problem is that the body struggles to absorb it. Because magnesium oxide is poorly soluble in water, most of it never crosses the intestinal wall into the bloodstream. Studies have consistently measured its fractional absorption rate at around 4% — meaning roughly 96% of what you swallow passes through unused.
That unabsorbed magnesium sitting in the large intestine is exactly what makes it an effective laxative. It draws water into the bowel, softening stools and stimulating bowel movements. This makes magnesium oxide genuinely useful for constipation and acid reflux — but a poor choice if your goal is to raise magnesium levels.
A 2017 randomised trial in JAMA Internal Medicine found magnesium oxide was no better than placebo for reducing nocturnal leg cramps — almost certainly because so little of it reaches circulation.
What Is Magnesium Glycinate?
Magnesium glycinate (also called magnesium bisglycinate or chelated magnesium) is magnesium bound to glycine, an amino acid. This chelated bond changes how the magnesium is processed in the gut — the glycine acts as a carrier, helping it cross the intestinal wall more efficiently and potentially via a separate dipeptide transport pathway.
Research consistently shows organic chelated forms of magnesium are better absorbed than inorganic forms like oxide. A 2005 study in Magnesium Research testing ten magnesium compounds found organic forms outperformed inorganic ones across the board for bioavailability. The exact absorption percentage varies by individual, dose, and gut health — but the advantage over oxide is consistent across studies.
It also produces far less osmotic activity in the colon, which is why it doesn’t cause the laxative effects associated with oxide.
Glycine itself has independent benefits. It’s an inhibitory neurotransmitter that promotes GABA activity in the brain, with calming and sleep-supporting properties. Magnesium glycinate therefore delivers two compounds with overlapping benefits in a single supplement — which is why it’s the preferred form for sleep, anxiety, and daily use.
The Absorption Gap — Why It Matters in Practice
The low absorption of magnesium oxide isn’t just a number — it determines how much magnesium actually reaches your cells.
If you take a 400mg elemental magnesium oxide supplement, only a small fraction enters your bloodstream. The rest stays in your gut. This is why many people take magnesium oxide supplements and notice little benefit beyond a laxative effect — the magnesium isn’t reaching the tissues where it does its work.
Magnesium glycinate delivers substantially more to circulation per dose, despite containing less elemental magnesium per gram. The higher absorption rate compensates for the lower elemental content — and does so without the digestive side effects.
Side Effects: How They Compare
The side effect profiles of these two forms are quite different, and this is often the deciding factor for people who have tried oxide and switched.
Magnesium oxide commonly causes loose stools, diarrhoea, and abdominal cramping at doses intended to meaningfully raise magnesium levels. This is its osmotic mechanism working as designed — which makes it useful as a laxative, but poorly tolerated for daily supplementation. At higher doses, nausea is also possible.
Magnesium glycinate has an excellent gastrointestinal tolerance record. Because it is absorbed via amino acid transporters in the small intestine, little unabsorbed magnesium reaches the colon. Most people tolerate it well even at higher doses over extended periods. At very high doses, mild sedation is the most commonly reported effect — which is generally a feature rather than a problem at standard supplemental amounts, given that both magnesium and glycine have calming properties.
When Magnesium Oxide Is the Right Choice
To be fair, magnesium oxide isn’t useless. It’s appropriate when:
- You need relief from constipation — its osmotic laxative effect is well-established and works quickly
- You have acid reflux or heartburn — it neutralises stomach acid effectively as an antacid
- You’re using it under medical supervision for a specific clinical purpose
For these applications, its low systemic absorption rate is irrelevant — the effect you want happens in the gut, not the bloodstream. What it’s not suited for: raising magnesium levels, supporting sleep, reducing anxiety, improving muscle function, or addressing a deficiency.
When Magnesium Glycinate Is the Right Choice
Magnesium glycinate is the better option for most people taking magnesium as a daily supplement. It’s particularly well-suited for:
- Correcting or preventing magnesium deficiency
- Supporting sleep quality and relaxation
- Reducing anxiety and the physiological stress response
- Muscle recovery and reducing cramping
- Anyone who has experienced digestive issues with other forms
Because it’s well-tolerated and doesn’t cause loose stools, it’s suitable for long-term daily use in a way that oxide is not for most people.
If you’re looking for a magnesium glycinate supplement in the UK, our Magnesium Glycinate uses pure bisglycinate — not a “glycinate complex” blended with cheaper forms — so the absorption advantage is preserved. It’s vegan-certified, made in the UK, independently tested at Campden BRI, and contains no artificial fillers or flow agents.
For a broader comparison across all magnesium forms, see our guide to chelated magnesium and our magnesium glycinate vs citrate comparison.
Watch Out for “Magnesium Glycinate Complex”
Some products are labelled “magnesium glycinate complex” or “magnesium chelate complex.” These often blend magnesium glycinate with cheaper forms such as magnesium oxide or carbonate to reduce manufacturing costs — but the label doesn’t always disclose the ratio.
If you’re buying magnesium glycinate specifically, check that the supplement facts panel lists only magnesium glycinate (or magnesium bisglycinate) as the magnesium source. Any blend that includes oxide or carbonate will have lower effective bioavailability than pure glycinate.
Dosage
The NHS recommends 300mg/day for men and 270mg/day for women of total magnesium. The EU’s tolerable upper level for supplemental magnesium is 250mg/day from supplements.
Note that this refers to elemental magnesium — the actual magnesium content — not the weight of the compound. Always check the “elemental magnesium” figure on the label, not the total compound weight.
FAQ
Q: Is magnesium glycinate better than magnesium oxide?
A: For most supplementation purposes, yes. Magnesium glycinate is substantially better absorbed, significantly gentler on the digestive system, and better suited for raising and maintaining magnesium levels. Oxide is only preferable for its specific uses as a laxative or antacid.
Q: Which magnesium form is better for sleep?
A: Magnesium glycinate. Magnesium supports sleep partly by activating GABA receptors — GABA is the brain’s primary inhibitory neurotransmitter, and higher GABA activity promotes the transition from wakefulness to sleep. The glycine component adds to this: glycine is itself an inhibitory amino acid that has been shown to lower core body temperature slightly, a key physiological trigger for sleep onset. Magnesium oxide, by contrast, is so poorly absorbed that very little magnesium reaches systemic circulation — making it ineffective for sleep support.
Q: Which magnesium is better for anxiety?
A: Magnesium glycinate. One of magnesium’s key mechanisms in the nervous system is blocking NMDA receptors — a class of glutamate receptor that, when overactive, drives neurological hyperexcitability. Adequate magnesium acts as a physiological brake on this system, reducing the wired, anxious quality that often accompanies deficiency. Glycine compounds this effect through GABA activation. Because oxide delivers so little magnesium to systemic circulation, it is unlikely to have a meaningful effect on anxiety at typical doses.
Q: Which is better for muscle cramps?
A: Magnesium glycinate. Magnesium regulates muscle contraction by competing with calcium at the cellular level — calcium triggers contraction, while magnesium facilitates relaxation. When magnesium levels are low, muscles are more prone to sustained or involuntary contractions. For this mechanism to work, magnesium needs to reach circulation and muscle tissue — which glycinate does. Magnesium oxide was directly tested in a 2017 randomised trial published in JAMA Internal Medicine and found to be no better than placebo for nocturnal leg cramps, almost certainly because so little of it is absorbed.
Q: Why is magnesium oxide in so many supplements if it’s poorly absorbed?
A: It’s cheap to produce and has a high elemental magnesium content by weight, making it look impressive on a label. Many budget multivitamins use it to hit a high milligram number, even though little of it is absorbed.
Q: Can I take magnesium oxide and glycinate together?
A: You can, but there’s rarely a reason to. If you want digestive support, oxide can help with that. If you want to raise magnesium levels, glycinate is the better choice. Using both simultaneously increases the risk of exceeding the supplemental upper limit and causing digestive discomfort.
Q: Is magnesium oxide safe?
A: At standard doses for its intended purposes — constipation or heartburn — it’s safe for most healthy adults. The main risks are digestive side effects at higher doses, and the risk of magnesium excess in people with kidney disease. It shouldn’t be used as a substitute for an absorbable magnesium supplement.
Q: How do I know if my magnesium supplement contains oxide?
A: Check the supplement facts panel under “magnesium source” or “form.” Pure magnesium glycinate will explicitly state glycinate or bisglycinate. If it says “magnesium oxide,” “magnesium” without specifying the form, or “magnesium complex,” the bioavailability is likely lower than a pure chelated product. A useful rule of thumb on the numbers: pure magnesium bisglycinate delivers around 11% elemental magnesium by weight. If a product labelled as glycinate shows significantly more than 15% elemental magnesium per gram of compound, it almost certainly contains oxide or another inorganic form blended in — regardless of what the product name says.
References
- Coudray C, et al. (2005). Study of magnesium bioavailability from ten organic and inorganic Mg salts. Magnesium Research, 18(4), 215–223.
- Maor NR, et al. (2017). Effect of magnesium oxide supplementation on nocturnal leg cramps. JAMA Internal Medicine, 177(5), 617–623.
- Schuchardt JP & Hahn A. (2017). Intestinal Absorption and Factors Influencing Bioavailability of Magnesium Supplements. Current Nutrition and Food Science, 13(4).
- Schuette SA, et al. (1994). Bioavailability of magnesium diglycinate vs magnesium oxide in patients with ileal resection. Journal of Parenteral and Enteral Nutrition, 18(5), 430–435.
- NHS. Vitamins and minerals — Magnesium.
This article is for informational purposes only and does not constitute medical advice. Consult a qualified healthcare professional before starting any supplement.
