Guide
Magnesium glycinate vs oxide
Oxide is about 60% elemental magnesium by weight but performed no better than placebo in a 60-day trial. Glycinate is lower in elemental magnesium and tied with oxide in the one direct comparison. Here's the full picture.
Magnesium oxide has the highest elemental magnesium of any common form — about 60% by weight — while glycinate holds about 11–14%. That makes oxide look like the better deal per milligram on the label, but it has also shown the weakest absorption in the trials that measured it directly, including one that found no difference from placebo at all.
The two forms, side by side
| Magnesium glycinate | Magnesium oxide | |
|---|---|---|
| Formula | C4H8MgN2O4 | MgO |
| Molar mass | 172.42 g/mol (PubChem CID 84645) | 40.30 g/mol (PubChem CID 14792) |
| Elemental magnesium by weight | ~11–14% | ~60% |
| Typically sold for | General daily supplementation, higher cost per dose | Cheap, high-mg-per-capsule supplements; also used in antacids and as an osmotic laxative |
| Best head-to-head result | Equal to oxide in the whole-group result of the only direct trial; didn't outperform it | No different from placebo in a 60-day healthy-adult trial |
| Watch out for | Higher price per gram of actual elemental magnesium | The large "mg" number on the label overstates what's actually absorbed |
Why oxide's big number is misleading
Because oxide is about 60% elemental magnesium, a "500 mg magnesium oxide" capsule contains roughly 300 mg of actual magnesium — a genuinely large elemental dose for the price. The catch is absorption: a 2003 randomized, double-blind, placebo-controlled trial gave 46 healthy adults 300 mg of elemental magnesium a day as citrate, an amino-acid chelate, or oxide for 60 days. Oxide supplementation showed no difference from placebo on any measure tested. The organic forms (citrate and the chelate) both showed significantly greater absorption than oxide by 24-hour urinary magnesium excretion.
The one trial directly comparing glycinate (as magnesium diglycinate) to oxide told a more nuanced story. It gave a single 100 mg dose to 12 patients who had part of their small intestine surgically removed, and found the whole-group absorption was essentially tied: 23.5% for the glycinate chelate versus 22.8% for oxide. The often-repeated claim that glycinate clearly outperforms oxide comes from a post hoc subgroup of just four patients — the ones who absorbed oxide worst — not from the trial's main result.
What each is typically used for
Oxide shows up in two very different products: cheap general-purpose magnesium capsules, where a high "mg" figure is a selling point, and drug products — antacids and osmotic laxatives — where poor absorption is the whole point, because the unabsorbed magnesium draws water into the bowel. That second use is a different product at a different dose than a daily supplement capsule.
Glycinate doesn't have a laxative use case. It's marketed on being gentler than oxide and better absorbed, and the evidence above doesn't clearly support either claim over oxide at typical doses — the honest comparison is closer than the marketing suggests, with citrate outperforming both in the trial that tested all three.
Side effects
Oxide's poor absorption is directly connected to its best-known side effect: what isn't absorbed tends to loosen stools, which is why it's used deliberately as a laxative at higher doses. Glycinate is generally considered milder on the gut at an equivalent elemental dose, though there's no controlled trial directly comparing GI tolerability between the two forms in capsule doses rather than laxative doses.
Questions people ask
What is the difference between magnesium oxide and magnesium glycinate?
Oxide is a simple inorganic salt, about 60% elemental magnesium, cheap to produce, and shown to perform no better than placebo in one 60-day healthy-adult trial. Glycinate is an organic chelate, about 11–14% elemental magnesium, and tied with oxide in the one direct absorption comparison between them.
Is magnesium oxide the same as magnesium glycinate?
No. They're different compounds with very different elemental percentages and different typical uses — oxide is also sold as a laxative and antacid, which glycinate is not.
Which is better, magnesium oxide or magnesium glycinate?
Neither has a clear win. Citrate outperformed both in the one trial that tested all three head-to-head; glycinate and oxide were statistically tied in the one trial comparing them directly. Oxide gives more elemental magnesium per dollar; glycinate is generally considered easier on the stomach at a matched dose.
Can you take magnesium oxide and magnesium glycinate together?
There's no specific hazard in doing so, but there's also no benefit beyond taking either form at the combined elemental total, which should stay under the 350 mg/day supplemental upper limit for most adults.
Is magnesium glycinate better than magnesium oxide for someone prone to loose stools?
It's the generally assumed choice, and it's consistent with oxide's known laxative use at higher doses. There isn't a controlled trial proving the gap at matched everyday doses, so treat it as a reasonable thing to try rather than an established fact.
Whats the difference between magnesium oxide and magnesium glycinate in price?
Oxide is almost always cheaper per gram of elemental magnesium, because the raw compound is inexpensive and delivers a lot of magnesium per gram. Glycinate typically costs more per 100 mg of elemental magnesium, which is the fair way to compare prices across forms rather than comparing bottle price alone.
Related reading
Sources
- Mg citrate found more bioavailable than other Mg preparations in a randomised, double-blind study In 46 healthy adults taking 300 mg a day for 60 days, citrate raised blood magnesium most; an amino-acid chelate did not beat it.
- Bioavailability of magnesium diglycinate vs magnesium oxide in patients with ileal resection In 12 people after bowel surgery, one 100 mg dose of glycinate was absorbed about the same as oxide overall.
- Perspective: Call for Re-evaluation of the Tolerable Upper Intake Level for Magnesium Supplementation in Adults In 7 of 10 trials between 128 and 1,200 mg a day, magnesium caused no more diarrhea than the comparison.
*These statements have not been evaluated by the Food and Drug Administration. This product is not intended to diagnose, treat, cure, or prevent any disease.
