Magnesium oxide

Grades and forms

Magnesium oxide carries 60.3 % elemental magnesium on an anhydrous basis. That is the highest content of the magnesium forms compared on this page. A label claim of 100 mg magnesium needs about 166 mg of magnesium oxide before assay correction. This is why manufacturers choose it for compact tablets and capsules.

Magnesium oxide powder is commonly traded as light and heavy types. Both are MgO, but they differ in bulk density and particle structure. Light types are voluminous and dust readily. Heavy types are denser and fill capsules and tablet dies more compactly. Granulated grades intended for direct compression are also used.

The choice depends on four things:

• the format and the fill volume or tablet mass available
• the flow your equipment needs
• whether the material is blended dry or granulated
• the purity specification your quality team works to

The specification sheet and certificate of analysis state the assay, particle size, loss on ignition and contaminant limits. SoleChem sends both with its quote, from ISO 9001:2015, ISO 22000:2018 and GMP+ certified operations. Ask for the grade you need in the quote form.

EU regulatory status

Magnesium oxide is listed as a mineral substance in Annex II of Directive 2002/46/EC. It may therefore be used as a source of magnesium in food supplements; magnesium itself appears in Annex I. The same form is listed in Annex II of Regulation (EC) No 1925/2006 for addition to other foods.

As E530, magnesium oxide is also a food additive under Regulation (EC) No 1333/2008. Food additive specifications are set in Regulation (EU) No 231/2012. Additive use and nutrient use rest on different legal bases, so confirm which one your formula relies on.

Maximum amounts of magnesium in food supplements are not harmonised at EU level. Member states set them nationally, and they differ. See maximum levels by EU country and the overview of permitted vitamin and mineral forms. Any claim wording must come from the EU Register of nutrition and health claims; the health-claims guide explains the rules.

Formulation notes

At 60.3 % magnesium, each 100 mg of magnesium on the label needs about 166 mg of magnesium oxide. A 200 mg claim needs about 332 mg. A 375 mg claim needs about 622 mg. These figures assume pure anhydrous material. Correct them with the assay on the certificate of analysis and add any overage your stability data supports. The elemental content calculator and overage calculator do this arithmetic.

Magnesium oxide is practically insoluble in water and dissolves in dilute acids. In drink powders it stays in suspension rather than dissolving. Expect sediment and a chalky mouthfeel. A soluble form such as magnesium citrate suits clear drinks better.

It is a basic oxide. On storage it slowly takes up moisture and carbon dioxide from air, forming hydroxide and carbonate at the particle surface. Keep containers closed and control humidity in blending rooms. In blends with acids such as ascorbic acid or citric acid, moisture can start neutralisation. Confirm compatibility in stability trials, especially for chewable and effervescent formats.

Light grades flow poorly and compress badly on their own. They usually need granulation, a glidant such as silicon dioxide, or replacement with a dense grade. Heavy and granulated grades fill capsules more efficiently. The small mass needed per milligram of magnesium keeps tablets small.

How magnesium oxide compares to other forms

Organic salts can be supplied as hydrates, which puts their real content below the anhydrous figure. The certificate of analysis gives the actual assay. For the trade-offs in detail, see magnesium oxide vs magnesium citrate and the magnesium forms guide.