Magnesium stearate

Magnesium stearate (CAS 557-04-0) is the magnesium salt of stearic acid, nominally Mg(C18H35O2)2. Commercial grades are mixtures of magnesium salts of fatty acids, mainly stearate and palmitate. Their composition depends on the fatty acid source, which may be vegetable or animal.

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What is magnesium stearate used for?

Its main role is as a lubricant in tablet compression and capsule filling. It reduces friction at the die wall and helps ejection, which prevents sticking and picking on punches. It is usually added at low levels, often in the range of about 0.25–2% w/w, and blended in as a final step. It also appears in cosmetics and as a food additive (E470b, magnesium salts of fatty acids).

Which quality grades and specifications apply?

Pharmaceutical grades are supplied to monographs in the EP, USP–NF and Japanese Pharmacopoeia. These monographs set limits on fatty acid composition, magnesium content, loss on drying, microbial quality and specified impurities. Procurement teams commonly ask for:

• a CoA confirming compliance with the relevant monograph
• a statement of source (vegetable or animal), and TSE/BSE documentation where animal-derived material is used
• an SDS and, where relevant, REACH registration status

Physical properties such as particle size and specific surface area are not always fully defined by monographs. They can still differ between suppliers and affect performance.

Why can magnesium stearate affect product performance?

Magnesium stearate is hydrophobic. Excessive amounts or over-long blending can coat other particles, a problem known as over-lubrication. This can reduce tablet hardness and slow disintegration and dissolution. Because of this, blending time and supplier changes are usually controlled carefully during formulation development and scale-up.

Related concepts

Magnesium stearate is closely linked to other excipients such as glidants (e.g. colloidal silicon dioxide) and alternative lubricants (e.g. sodium stearyl fumarate and stearic acid). Related topics include pharmacopoeial compliance, excipient supplier qualification and tablet dissolution testing.