Short answer
Nobody knows which is better, because the two have barely been compared head to head. Citrate has more absorption data: in small studies it was absorbed better than magnesium oxide. Glycinate has far less human data. Both count toward the 350 mg daily upper limit for supplemental magnesium, a limit that exists because too much causes diarrhea.
What the absorption studies show
The NIH Office of Dietary Supplements (ODS) says small studies found the aspartate, citrate, lactate and chloride forms are absorbed more completely than magnesium oxide and magnesium sulfate. In a 1990 study, citrate was more soluble and better absorbed than oxide. A 2003 trial gave 46 healthy adults 300 mg a day of an amino-acid chelate, citrate, oxide or placebo for 60 days. Both chelate and citrate beat oxide on urine measures, and citrate raised blood magnesium the most.
A 2021 systematic review of 14 studies found organic forms more bioavailable than inorganic ones, with absorption depending on dose.
Where glycinate fits
Human data on glycinate are thin. A 1994 trial in 12 patients who had part of their small intestine removed found absorption from magnesium diglycinate was no different from oxide for the group as a whole, though it was higher in the four patients who absorbed oxide worst, and it was better tolerated.
The authors of a 2025 trial put the gap plainly: clinical evidence directly comparing absorption of the forms remains limited. The same 2021 review concluded that all the forms it covered can maintain normal levels in healthy people without a prior deficit.
The laxative question
Magnesium citrate is also a drug. In US pharmacies it is sold as an over-the-counter saline laxative for occasional constipation, and its label says it generally produces a bowel movement in half an hour to 6 hours.
ODS explains that the laxative effect of magnesium salts comes from unabsorbed salt pulling water into the intestine. Its list of the forms most commonly reported to cause diarrhea is carbonate, chloride, gluconate and oxide. The National Academies, which set the intake limits, wrote that it is not known whether all magnesium salts behave the same way.
Why the 350 mg limit exists
The Tolerable Upper Intake Level for supplemental magnesium is 350 mg a day for adults. The recommended intake is 420 mg for men and 320 mg for women over 30. The limit looks lower because it counts only supplements and medicines, not food.
The National Academies chose diarrhea as the deciding harm and found it starting at 360 mg a day from non-food sources. ODS adds that kidney disease raises the risk of magnesium toxicity, and that magnesium can interact with some osteoporosis drugs and antibiotics. Ask your doctor or pharmacist before starting.
Related questions
Is magnesium glycinate better than citrate for sleep or anxiety?
That has not been tested. The 2025 sleep trial of bisglycinate compared it with a placebo, not with citrate, and we found no trial comparing the two forms for anxiety.
Does magnesium from food count toward the 350 mg limit?
No. ODS says the upper limit covers only magnesium from supplements and medicines, because the kidneys clear extra magnesium from food in healthy people.
Is magnesium oxide a bad form?
It is less well absorbed. In the 2003 trial, magnesium oxide produced no differences compared with placebo.
Read next
Sources
- NIH Office of Dietary Supplements: Magnesium, Fact Sheet for Health Professionals (updated January 2026)
- Institute of Medicine (National Academies): Dietary Reference Intakes for Calcium, Phosphorus, Magnesium, Vitamin D, and Fluoride, Chapter 6, Magnesium (1997)
- Walker et al., Magnesium Research: Mg citrate found more bioavailable than other Mg preparations in a randomised, double-blind study (2003)
- Pardo et al., Nutrition: Bioavailability of magnesium food supplements, a systematic review (2021)
- Schuette et al., JPEN: Bioavailability of magnesium diglycinate vs magnesium oxide in patients with ileal resection (1994)
Published 2026-10-07 · Last checked 2026-10-07
Educational content, not medical advice. Made for the United States.