Magnesium Oxide vs Glycinate (2026): How They Differ
For everyday magnesium, glycinate is the gentler choice; for constipation, oxide is the form with trials. In the one trial that compared them at the same dose, both were absorbed about the same, and glycinate was better tolerated by every patient. Oxide costs far less per mg at list price.
The verdict
Pick glycinate if you want a daily magnesium that is easy on the stomach; it has not been shown to absorb better than oxide. Pick oxide if you want the lowest cost per mg or a laxative effect: much of it stays in the gut and draws water into the bowel. Both costs are per 300 mg of magnesium at each brand’s list price; prices on Amazon differ.
A comparison of labels, studies and cost, not medical advice. Prices are each brand’s own list price, checked October 14, 2026; Amazon’s can differ.
Oxide vs glycinate, side by side
| Oxide | Glycinate | |
|---|---|---|
| Magnesium a serving | 250 mg in 1 tablet | 200 mg in 2 capsules |
| Magnesium in the compound | About 60% | About 14% |
| Absorbed, same dose, same people | 22.8% | 23.5% (no real difference; Schuette 1994, 12 people) |
| Stomach | Less well tolerated in that trial; a laxative at high doses | Better tolerated by all 12 patients (Schuette 1994) |
| Main use | Constipation, heartburn | Everyday magnesium |
| Testing list | None found (its page shows a USP mark; USP’s list does not carry it) | USP Verified |
| Per 300 mg of magnesium, list price | $0.071 | $1.10 |
| Buy | Check price | Check price |
The one trial that compared them
Twelve people who had part of their small bowel removed took a 100 mg dose of each form, with a tracer to measure what they absorbed. Overall the two were the same: 23.5% of the glycinate and 22.8% of the oxide. In the 4 people who absorbed oxide worst, glycinate did twice as well (23.5% vs 11.8%), and everyone tolerated glycinate better (Schuette 1994). It is one small trial in people with a gut problem, so it says little about healthy adults.
Small studies found the citrate, aspartate, lactate and chloride forms absorbed better than oxide and sulfate (NIH). NIH says nothing about glycinate either way. Another study measured only about 4% of oxide absorbed in healthy volunteers (Firoz 2001). It used a different method (extra magnesium in urine, at a higher dose) and did not test glycinate, so its number cannot be set beside Schuette’s.
Why oxide looks better on the label
Oxide is about 60% magnesium by weight, against about 14% for glycinate, so a small oxide tablet can print a big number: a Nature Made oxide tablet holds 250 mg of magnesium. The label shows what is in the tablet, not what your body takes up. Our oxide vs citrate page charts label vs absorbed for one oxide tablet.
The magnesium that is not absorbed pulls water into the bowel. That is why oxide works for constipation: in two trials, 68% of people improved on it against 19% on a placebo, at doses far above a supplement serving (van der Schoot 2023). More on that in magnesium oxide vs citrate.
If you choose glycinate
Our pick is above. If you want a second option, these are the glycinates we link, one line each; costs are per 300 mg of magnesium at list price. Every glycinate we price is on what each magnesium glycinate costs.
- Thorne Magnesium Bisglycinate: on a testing list (NSF/ANSI 173); 120 mg a capsule, $0.72.Check price
- Doctor’s Best Magnesium Glycinate: lower price, no testing list; 200 mg in 2 tablets, $0.30.Check price
- Vitamin Shoppe Magnesium Glycinate 400 mg: 135 tablets, 133 mg each. Take 1 or 2: the 3-tablet serving (400 mg) is over the limit. Not on a testing list.Check price
- KAL Magnesium Glycinate 350: 350 mg in 4 capsules, the whole daily limit; no testing list, no list price.Check price
- BulkSupplements Magnesium Glycinate Powder: a glycinate + oxide blend, $0.19.Check price
Safety
- The upper limit for adults is 350 mg a day of magnesium from supplements and medicines. Food does not count toward it (NIH).
- If you have kidney disease, ask your doctor before you take magnesium. Weak kidneys cannot clear the extra, so it can build up (NIH).
- Antibiotics: take tetracyclines (such as doxycycline) or quinolones (such as ciprofloxacin) at least 2 hours before or 4 to 6 hours after magnesium (NIH).
Frequently asked questions
Is magnesium glycinate better absorbed than oxide?
Not proven. The one trial that compared them, in 12 people who had part of their small bowel removed, found no overall difference (23.5% vs 22.8% absorbed). Glycinate did better only in the 4 people who absorbed oxide worst (Schuette 1994). Another study, in healthy people, measured only about 4% of oxide absorbed, but it did not test glycinate and measured absorption a different way (Firoz 2001).
Why do so many supplements use magnesium oxide?
It is cheap and packs the most magnesium into a small tablet: oxide is about 60% magnesium by weight, against about 14% for glycinate. So the label number looks big. Little of it is absorbed, which is why oxide works as a laxative.
What is magnesium oxide good for?
Constipation and heartburn. In two constipation trials, 68% of people improved on oxide against 19% on a placebo (van der Schoot 2023). Those trials used far more than a supplement dose, so use it that way only with a doctor’s advice.
How much magnesium oxide equals one glycinate capsule?
Nobody can say. No study gives the share of glycinate that is absorbed, so there is no conversion. Taking more oxide to make up for it mostly means looser stools.
Is glycinate easier on the stomach than oxide?
Yes, in the one small trial that compared them: all 12 patients tolerated glycinate better at the same 100 mg dose (Schuette 1994). No large trial has checked this.
Why does one study say 4% of oxide is absorbed and another say 22.8%?
They measured different things in different people. Firoz 2001 gave healthy volunteers about 250 mg a day and measured the extra magnesium in their urine: about 4% for oxide. Schuette 1994 gave 12 patients after bowel surgery a single 100 mg dose with a tracer: 22.8% for oxide and 23.5% for glycinate. Neither number is the “true” figure for everyone; what matters here is that in the same people, at the same dose, the two forms came out about the same.
Related
- Glycinate vs citrate: which to take · oxide vs citrate · L-threonate vs glycinate
- All magnesium forms compared · best magnesium glycinate · magnesium guide
Sources
- Schuette SA, Lashner BA, Janghorbani M. Bioavailability of magnesium diglycinate vs magnesium oxide in patients with ileal resection. JPEN. 1994. PubMed 7815675
- Firoz M, Graber M. Bioavailability of US commercial magnesium preparations. Magnes Res. 2001. PubMed 11794633
- van der Schoot A, et al. The effect of food, vitamin, or mineral supplements on chronic constipation in adults. Neurogastroenterol Motil. 2023. PubMed 37243443
- NIH Office of Dietary Supplements. Magnesium fact sheet for health professionals (updated January 6, 2026).
- Labels and prices: Nature Made, Doctor’s Best, Thorne, KAL and BulkSupplements pages, list prices checked October 14, 2026; Vitamin Shoppe’s serving from its Amazon listing. Testing lists searched October 14, 2026.