Magnesium Oxide vs Glycinate: Which to Take?

To raise your magnesium levels, glycinate is the safer bet: only about 4% of magnesium oxide was absorbed in one study¹. For constipation, oxide is the better choice, because unabsorbed magnesium draws water into the bowel² and it beat placebo in trials^{3, 4}.

Oxide also did no better than placebo in a 60-day absorption trial⁵, although the one direct glycinate comparison found similar absorption overall, with glycinate ahead in poor absorbers⁶. The twist is that key sleep and migraine trials gave oxide, not glycinate^{7, 8}, so neither form has proven benefits the other lacks. For what magnesium itself does, see its evidence page.

The difference is what the magnesium is bound to, and that changes how much dissolves and gets absorbed. Magnesium oxide is magnesium bound to oxygen, an inorganic salt. Magnesium glycinate is magnesium bound to the amino acid glycine, a chelate that trials also call diglycinate or bisglycinate^{6, 9}.

• Solubility: forms that dissolve well in liquid are absorbed more completely, and oxide dissolves poorly².
• Absorption route: part of magnesium diglycinate appears to be absorbed intact, probably through the gut's dipeptide transporters⁶.
• Gut effect: unabsorbed magnesium salts pull water into the bowel, which is why oxide acts as a laxative².
• Medical use: oxide is also sold as an antacid and a laxative, not only as a supplement¹⁰.

Comparing doses: on a Supplement Facts panel, the label must state the elemental magnesium, not the weight of the whole compound². So 400 mg on either label means 400 mg of magnesium. What differs is how much of it you absorb, which is low for oxide¹.

Note that 400 mg is above the 350 mg daily upper limit for magnesium from supplements in adults². Take that dose only on a clinician's advice.

Several forms, including citrate, chloride and an amino-acid chelate, were absorbed better than oxide. The one direct glycinate comparison found similar absorption overall, with glycinate ahead only in the poorest absorbers. Here is what the head-to-head studies found:

• Oxide vs chloride, lactate and aspartate: in healthy volunteers given about 21 mEq a day, fractional absorption of oxide was 4%, and the other three forms were absorbed significantly better¹.
• Oxide vs amino-acid chelate vs citrate: in a 60-day randomised trial in 46 healthy adults taking 300 mg a day, the chelate and citrate raised 24-hour urine magnesium more than oxide, and oxide was no different from placebo⁵.
• Oxide vs diglycinate: in 12 patients with part of the small intestine removed, a 100 mg dose was absorbed equally overall (23.5% vs 22.8%). In the 4 worst absorbers, diglycinate was absorbed twice as well (23.5% vs 11.8%), and every patient tolerated it better⁶.

A 2021 systematic review of 14 studies concluded that inorganic forms such as oxide appear less bioavailable than organic ones, and that absorption depends on dose¹¹. It also concluded that any magnesium supplement can maintain normal levels in healthy people without a deficit; that is less certain for older or ill people¹¹.

One trial points the other way. In a 1-month crossover in 41 healthy adults, oxide at 520 mg a day raised magnesium inside cells, while citrate at 296 mg a day did not significantly do so¹². The oxide dose was almost twice as high, so a bigger dose can partly offset poor absorption.

Neither has strong evidence, and the best-known positive trial used oxide. Glycinate is often sold for sleep, but it has one trial with a small effect.

• Oxide: 46 older adults with insomnia took 500 mg of magnesium a day as oxide tablets for 8 weeks. Insomnia scores, sleep efficiency and time to fall asleep improved more than on placebo⁷.
• Bisglycinate: 155 adults with poor sleep took 250 mg a day. Insomnia scores fell by 3.9 points versus 2.3 on placebo at week 4, a small effect (Cohen's d 0.2), with no change in other psychological outcomes⁹.

Pooled data are weak for all forms. A 2021 meta-analysis of 3 trials in 151 older adults found people fell asleep 17.4 minutes faster on magnesium, but the 16-minute gain in total sleep was not significant and evidence quality was low¹³. A 2026 review of 12 trials found inconsistent results and concluded magnesium is not supported as a routine insomnia treatment¹⁴.

If you try magnesium for sleep, glycinate is the gentler option at night; the evidence does not favour either form. Our pages on magnesium for insomnia and magnesium for sleep quality grade the full evidence.

For migraine, the trial evidence is for oral magnesium in general, and we found no published trial of glycinate for migraine. A 2016 meta-analysis pooled 10 oral magnesium trials in 789 people and found fewer and less intense migraines (odds ratios 0.20 and 0.27)¹⁵.

Oxide's own record is mixed. In 118 children aged 3 to 17, oxide at 9 mg per kg a day for 16 weeks reduced headache days, but the decline was not significantly different from placebo; headache severity was lower⁸.

That was a supervised trial dose. The upper limit for supplemental magnesium is 65 mg a day for children aged 1 to 3 and 110 mg for ages 4 to 8², so give a child magnesium for headaches only on a doctor's advice.

So the form matters less than taking a tolerable dose daily. Our pages on magnesium for migraine prevention and migraine cover the doses used.

Oxide is the one to use for constipation, because its poor absorption is the point. Unabsorbed magnesium salts draw water into the gut and speed it up².

• 34 women with mild to moderate constipation: 0.5 g of oxide 3 times a day for 28 days improved symptoms in 70.6%, versus 25.0% on placebo³.
• 90 adults with chronic constipation: 1.5 g of oxide a day for 28 days worked for 68.3%, about as well as senna (69.2%) and far better than placebo (11.7%), with no serious side effects⁴.

These doses are in grams of the compound, used as a laxative, not a daily supplement. Oxide is about 60% magnesium by weight (414 mg of oxide supplies 250 mg of magnesium)⁷, so 1.5 g provides roughly 900 mg of magnesium. That is well above the 350 mg upper limit, which covers medicines as well as supplements², so use laxative doses only under medical supervision. For heartburn, oxide is used as an antacid¹⁰; glycinate is not sold for either purpose. See magnesium for constipation for the wider evidence.

The main downsides are diarrhea and, in people with weak kidneys, a build-up of magnesium in the blood. Oxide is one of the forms most often reported to cause diarrhea, with carbonate, chloride and gluconate². High doses of any form can also cause nausea and abdominal cramping².

The bigger risk is hypermagnesemia (too much magnesium in the blood). In 320 hospital patients prescribed oxide, 23% developed it¹⁰. The risk factors were:

• Reduced kidney function: eGFR of 55.4 mL/min or lower¹⁰.
• High blood urea nitrogen: 22.4 mg/dL or higher¹⁰.
• Higher dose: 1,650 mg of oxide a day or more¹⁰.
• Longer use: 36 days or more¹⁰.

A case series described 4 people over 65 with kidney problems who developed severe hypermagnesemia on oxide laxatives, and one died¹⁶. A 2021 review advised taking account of dose, blood levels and drug interactions, especially in older people and those with kidney impairment¹⁷.

So avoid oxide without medical advice if you have chronic kidney disease, are older and on a daily laxative dose, or cannot easily report symptoms. These risks come from the magnesium, so glycinate is not a safe workaround in kidney disease. Very large doses of magnesium laxatives or antacids, typically over 5,000 mg a day, have caused toxicity².

Magnesium in any form interacts with the same medicines²:

• Bisphosphonates (such as alendronate): take magnesium at least 2 hours before or after².
• Tetracycline and quinolone antibiotics: take the antibiotic at least 2 hours before or 4 to 6 hours after magnesium².
• Diuretics: loop and thiazide diuretics increase magnesium loss in urine, while potassium-sparing diuretics such as spironolactone reduce it².
• Long-term proton pump inhibitors: they can lower magnesium levels after more than a year of use².

Our guide to what not to take with magnesium covers the full list.

Doctors usually recommend oxide because it is cheap, easy to take and works as a laxative and antacid^{17, 10}. It is also about 60% magnesium by weight, so a small tablet carries a full dose⁷.

We found no evidence that oxide is the preferred form for heart patients. A placebo-controlled trial in 187 patients with coronary artery disease used 365 mg a day of magnesium citrate for 6 months and improved exercise time and chest pain¹⁸. In healthy adults, the high-dose oxide crossover described above raised cell magnesium and lowered LDL cholesterol¹².

Choose by your goal, and keep the supplemental dose at or under 350 mg a day unless a clinician advises more²:

• You want to correct low intake or a deficiency: glycinate, or another well-absorbed form, because oxide absorption can be as low as 4%¹. See magnesium for deficiency.
• You have constipation: oxide, the form tested in placebo-controlled constipation trials^{3, 4}.
• Magnesium upsets your gut: glycinate, which all 12 patients tolerated better than oxide in a small crossover trial in people with bowel resections⁶, and which is not among the forms most often reported to cause diarrhea².
• You want it for sleep or migraine: either; the trials do not show one form beating the other, and the sleep evidence is weak^{14, 15}.
• You have kidney disease: neither, until your doctor has checked your levels¹⁶.

Food comes first: our list of magnesium-rich foods shows how to reach your daily need from diet. If you are also weighing citrate, read magnesium glycinate vs citrate, and for timing, when to take magnesium.

Related conditions

• Magnesium Deficiency
• Chronic Constipation
• Insomnia
• Migraine Disorder
• Chronic Kidney Disease

We searched PubMed for head-to-head absorption studies of magnesium oxide and glycinate (also diglycinate, bisglycinate and amino-acid chelate), and for randomised trials and systematic reviews of each form for sleep, migraine and constipation, plus safety data on hypermagnesemia. We read the NIH Office of Dietary Supplements fact sheet for absorption, upper limits and interactions.

We read 20 study records and cited 17, prioritising systematic reviews and randomised trials in people. The main gap: almost no trial compares oxide and glycinate directly for a health outcome, and we found no glycinate trial for migraine or constipation. Researched October 2026.

1. Firoz M, et al. Bioavailability of US commercial magnesium preparations. Magnes Res. 2001;14(4):257-62. https://pubmed.ncbi.nlm.nih.gov/11794633/
2. National Institutes of Health, Office of Dietary Supplements. Magnesium: Fact Sheet for Health Professionals. https://ods.od.nih.gov/factsheets/Magnesium-HealthProfessional/
3. Mori S, et al. A Randomized Double-blind Placebo-controlled Trial on the Effect of Magnesium Oxide in Patients With Chronic Constipation. J Neurogastroenterol Motil. 2019;25(4):563-575. doi:10.5056/jnm18194 Our study summary.
4. Morishita D, et al. Senna Versus Magnesium Oxide for the Treatment of Chronic Constipation: A Randomized, Placebo-Controlled Trial. Am J Gastroenterol. 2021;116(1):152-161. doi:10.14309/ajg.0000000000000942 Our study summary.
5. Walker AF, et al. Mg citrate found more bioavailable than other Mg preparations in a randomised, double-blind study. Magnes Res. 2003;16(3):183-91. https://pubmed.ncbi.nlm.nih.gov/14596323/ Our study summary.
6. Schuette SA, et al. Bioavailability of magnesium diglycinate vs magnesium oxide in patients with ileal resection. JPEN J Parenter Enteral Nutr. 1994;18(5):430-5. doi:10.1177/0148607194018005430
7. Abbasi B, et al. The effect of magnesium supplementation on primary insomnia in elderly: A double-blind placebo-controlled clinical trial. J Res Med Sci. 2012;17(12):1161-9. https://pubmed.ncbi.nlm.nih.gov/23853635/ Our study summary.
8. Wang F, et al. Oral magnesium oxide prophylaxis of frequent migrainous headache in children: a randomized, double-blind, placebo-controlled trial. Headache. 2003;43(6):601-10. doi:10.1046/j.1526-4610.2003.03102.x
9. Schuster J, et al. Magnesium Bisglycinate Supplementation in Healthy Adults Reporting Poor Sleep: A Randomized, Placebo-Controlled Trial. Nat Sci Sleep. 2025;17:2027-2040. doi:10.2147/NSS.S524348 Our study summary.
10. Wakai E, et al. Risk factors for the development of hypermagnesemia in patients prescribed magnesium oxide: a retrospective cohort study. J Pharm Health Care Sci. 2019;5:4. doi:10.1186/s40780-019-0133-7
11. Pardo MR, et al. Bioavailability of magnesium food supplements: A systematic review. Nutrition. 2021;89:111294. doi:10.1016/j.nut.2021.111294
12. Shechter M, et al. Comparison of magnesium status using X-ray dispersion analysis following magnesium oxide and magnesium citrate treatment of healthy subjects. Magnes Res. 2012;25(1):28-39. doi:10.1684/mrh.2012.0305
13. Mah J, et al. Oral magnesium supplementation for insomnia in older adults: a Systematic Review & Meta-Analysis. BMC Complement Med Ther. 2021;21(1):125. doi:10.1186/s12906-021-03297-z Our study summary.
14. Lopresti AL, et al. Magnesium Supplementation for Sleep in Adults: A Systematic Review of Randomized Controlled Trials. J Diet Suppl. 2026;23(5):553-581. doi:10.1080/19390211.2026.2719670
15. Chiu HY, et al. Effects of Intravenous and Oral Magnesium on Reducing Migraine: A Meta-analysis of Randomized Controlled Trials. Pain Physician. 2016;19(1):E97-112. https://pubmed.ncbi.nlm.nih.gov/26752497/ Our study summary.
16. Yamaguchi H, et al. Severe hypermagnesemia induced by magnesium oxide ingestion: a case series. CEN Case Rep. 2019;8(1):31-37. doi:10.1007/s13730-018-0359-5
17. Mori H, et al. Magnesium Oxide in Constipation. Nutrients. 2021;13(2). doi:10.3390/nu13020421
18. Shechter M, et al. Effects of oral magnesium therapy on exercise tolerance, exercise-induced chest pain, and quality of life in patients with coronary artery disease. Am J Cardiol. 2003;91(5):517-21. doi:10.1016/s0002-9149(02)03297-6