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    Ingredient guide · Ingredient reference

    Magnesium oxide 400 mg: check whether that is compound or elemental mass

    A 400 mg magnesium oxide statement differs from 400 mg elemental magnesium supplied as oxide. The distinction changes the amount you are actually comparing.

    Read the evidence ↓

    Supplement Science Insights Editorial Team

    Published September 23, 2026

    Two labels can describe different quantities

    Magnesium oxide is the compound MgO. On a Supplement Facts panel, a line declaring magnesium as magnesium oxide gives elemental magnesium. A medicine or front label may instead name the compound amount. Do not interpret an isolated 400 mg figure without its full context. [1][2]

    Purpose and tolerance matter

    Magnesium compounds can appear in nutritional supplements and medicines. The appropriate use follows the exact product and clinical purpose. ODS describes lower relative bioavailability for oxide than some more soluble forms, but that does not mean it contains no usable magnesium. Diarrhea, nausea and cramps can occur with high nonfood intake, and kidney impairment raises toxicity risk. [1]

    A stoichiometric example, not a substitute for the label

    Using approximate atomic masses of 24.3 for Mg and 16.0 for O, pure MgO is about 60.3% magnesium by mass. Thus 400 mg pure compound corresponds to about 241 mg magnesium. If the panel already declares 400 mg magnesium as oxide, do not multiply again: it is already the elemental amount. Verify the actual product declaration. [2][1]

    Hypothetical example — illustrative label or use scenario, not a prescription
    Hypothetical declarationInterpretation
    400 mg pure MgO compoundAbout241 mg magnesium by formula
    Magnesium400 mg(as oxide)400 mg elemental magnesium
    Unclear front-label400 mgRead the full panel
    Magnesium oxide 400 mg: check whether that is compound or elemental mass — original article outline diagram
    Original editorial diagram of this article’s sections; no product photograph or clinical finding is depicted.

    Sources for this article

    1. Magnesium - Health Professional Fact Sheet
      Source consulted: 2026-09-20. This is a reference resource, not a claim that we conducted the studies or tested a product.
    2. PubChem: Magnesium Oxide, CID14792
      Source consulted: 2026-09-20. This is a reference resource, not a claim that we conducted the studies or tested a product.