TL;DR: Magnesium supplements differ more in form than in dose. The same 500 mg capsule can deliver 302 mg of actual magnesium as the oxide or 41 mg as L-threonate — a seven-fold spread. Choose on elemental content and tolerance, then check the total against the 350 mg supplemental limit.

Magnesium is the ingredient where the label is most likely to mislead you, because almost every product is sold under the name of a compound rather than the mineral. A bottle marked “Magnesium 500 mg” and another marked “Magnesium Bisglycinate 500 mg” can differ by a factor of four in what they actually provide.

This page covers the forms, what the evidence supports about each, and how to read the number on the bottle. Intake matters for a measurable share of US adults: in NHANES 2017–March 2020, 57% of men and 52% of women consumed less than the Estimated Average Requirement, with mean intakes of 339 mg/day and 269 mg/day (Linus Pauling Institute, Oregon State University).

How much elemental magnesium each form contains

This table is arithmetic from molecular weights, not a claim requiring a citation. You can verify any row.

Higher elemental content is not automatically better. Oxide carries the most magnesium per milligram and is also the least soluble and the most likely to loosen stools. That trade-off is the whole reason multiple forms exist.

What each form is typically used for

A caution that applies to this whole table: most absorption comparisons test one or two forms against each other, in small groups, using urinary excretion rather than clinical outcomes. Ranking all ten forms by absorption would require evidence that does not exist.

Going deeper: glycinate vs oxide covers the absorption claims in detail, including where the widely quoted “4% vs 18.8%” figures come from.

How much you need, and the limit that is lower

The upper limit being lower than the requirement confuses almost everyone who notices it. The two numbers count different things.

The RDA counts magnesium from everything: food, water, supplements and medication. The Tolerable Upper Intake Level of 350 mg counts only supplements and magnesium-containing medicines such as some antacids and laxatives. Magnesium in food is not subject to it, because healthy kidneys clear the excess. The limit is set at the level “likely to pose no risk of diarrhea.”

Two things follow. A magnesium-containing antacid counts toward the 350 mg. And in our survey of 90 product labels, close to half of products on the market exceeded that limit in a single serving.

How to read a magnesium label

The front of the bottle names the compound. The Supplement Facts panel names the mineral. Only the second one tells you what you are taking.

• “Magnesium (as magnesium citrate) 100 mg” — 100 mg of elemental magnesium. This is the useful number.
• “Magnesium citrate 1,000 mg” — 1,000 mg of the compound. At roughly 16%, about 162 mg of magnesium.
• “% Daily Value” — calculated against 420 mg, so 50% DV means about 210 mg of elemental magnesium.

Where a label states only the compound weight, the table above converts it. Where it states both, the two should be consistent — in our label survey only 3 of 45 bisglycinate products and none of 45 oxide products gave both numbers.

Safety and interactions

Diarrhea is the first sign of too much. Between 19% and 40% of people taking magnesium supplements report diarrhea or stomach upset (Linus Pauling Institute). It is dose-dependent and more common with poorly soluble forms.

Reduced kidney function is the serious risk. Among patients taking daily magnesium oxide for constipation, 38% of those with CKD stage 4–5 had elevated serum magnesium and 19% met criteria for hypermagnesemia (Mori 2019). No patient in that study was symptomatic, but the association with kidney function was clear. Do not supplement magnesium with impaired kidney function without medical supervision.

Interactions run in two directions, and the fix differs:

• Magnesium blocks the drug — bisphosphonates, quinolone and tetracycline antibiotics. Separate the doses. The FDA alendronate label requires waiting at least 30 minutes; MedlinePlus advises 2 hours as a general rule for magnesium oxide.
• The drug depletes magnesium — proton pump inhibitors and loop or thiazide diuretics. Spacing does not help. The FDA issued a 2011 Drug Safety Communication on low magnesium after long-term PPI use, mostly beyond one year (summarised with citation by the Linus Pauling Institute).

Seek medical attention for severe or persistent diarrhea, muscle weakness, difficulty breathing, or an irregular heartbeat while taking magnesium.

Frequently asked questions

Which form of magnesium is best?

There is no form that is best for everyone, because the forms trade off against each other. Oxide gives the most magnesium per milligram and is cheapest; glycinate is chosen when tolerance matters; citrate has the clearest human absorption evidence relative to oxide. No trial has ranked all forms against clinical outcomes.

How do I know how much magnesium I am actually taking?

Read the Supplement Facts panel rather than the front label. US labelling requires elemental magnesium to be stated there. If only the compound weight appears, multiply by the percentage in the table above.

Can I take magnesium every day?

Many people do, within the 350 mg supplemental limit for adults. That limit counts supplements and magnesium-containing medicines but not food. Check whether an antacid or laxative you take also contains magnesium, and talk to a pharmacist if you take prescription medication.

Does magnesium help with sleep or cramps?

Both are common reasons people buy it, and both have less supporting evidence than the marketing suggests. We are reviewing the trial literature on these uses and will publish separate pages rather than summarise them here.

Sources

• Linus Pauling Institute, Oregon State University. Magnesium.
• NIH Office of Dietary Supplements. Magnesium — Health Professional Fact Sheet.
• Lindberg JS, et al. Magnesium bioavailability from magnesium citrate and magnesium oxide. J Am Coll Nutr. 1990;9(1):48-55. PMID 2407766
• Werner T, et al. Assessment of bioavailability of Mg from Mg citrate and Mg oxide. Magnes Res. 2019;32(3):63-71. PMID 32162607
• Schuette SA, et al. Bioavailability of magnesium diglycinate vs magnesium oxide in patients with ileal resection. JPEN. 1994;18(5):430-5. PMID 7815675
• Firoz M, Graber M. Bioavailability of US commercial magnesium preparations. Magnes Res. 2001;14(4):257-62. PMID 11794633
• Mori H, et al. Clinical features of hypermagnesemia in patients with functional constipation taking daily magnesium oxide. J Clin Biochem Nutr. 2019;65(1):76-81. PMC6667383
• MedlinePlus. Magnesium Oxide.
• US Food and Drug Administration. Drug Safety Communication: Low magnesium levels can be associated with long-term use of proton pump inhibitor drugs, 2011. The original FDA page has been retired; the communication is cited and summarised by the Linus Pauling Institute.
• NIH Dietary Supplement Label Database — label survey of 90 products, retrieved November 2026.

Elemental percentages calculated from standard molecular weights.

This page provides general information and does not replace advice from a qualified healthcare professional. Talk to your doctor or pharmacist before starting a supplement, particularly if you have kidney disease, are pregnant or breastfeeding, or take prescription medication.

Information only, not medical advice. Talk to a doctor or pharmacist before starting a supplement, particularly if you have kidney disease, are pregnant or breastfeeding, or take prescription medication.