Magnesium stearate
[CAS# 557-04-0]

Identification

• Classification: Pharmaceutical intermediate >> Heterocyclic compound intermediate >> Pyridine compound >> Chloropyridine
• Name: Magnesium stearate
• Synonyms: Octadecanoic acid magnesium salt
• Molecular Formula: 2(C₁₈H₃₅O₂)^.Mg
• Molecular Weight: 591.24
• CAS Registry Number: 557-04-0
• EC Number: 209-150-3
• SMILES: CCCCCCCCCCCCCCCCCC(=O)[O-].CCCCCCCCCCCCCCCCCC(=O)[O-].[Mg+2]

Properties

• Melting point: 200 ºC (Expl.)
• Solubility: water: Insoluble (Expl.)

Safety Data

• Hazard Symbols: GHS07 Warning
• Hazard Statements: H315-H319-H335-H413
• Precautionary Statements: P261-P264-P264+P265-P271-P273-P280-P302+P352-P304+P340-P305+P351+P338-P319-P321-P332+P317-P337+P317-P362+P364-P403+P233-P405-P501

Hazard Classification

Hazard	Class	Category Code	Hazard Statement
Eye irritation	Eye Irrit.	2	H319
Specific target organ toxicity - single exposure	STOT SE	3	H335
Skin irritation	Skin Irrit.	2	H315
Chronic hazardous to the aquatic environment	Aquatic Chronic	4	H413
Specific target organ toxicity - repeated exposure	STOT RE	2	H373
Specific target organ toxicity - repeated exposure	STOT RE	1	H372
Pyrophoric solids	Pyr. Sol.	1	H250

Discovory and Applicatios

Magnesium stearate is a magnesium salt of stearic acid, historically developed as one of the earliest metal soaps and lubricants in powder technology. Its production emerged in the late nineteenth to early twentieth century when fatty acid salts were explored for industrial uses. It is typically formed by reacting stearic acid with magnesium oxide or hydroxide, or by double displacement of sodium stearate with magnesium salts. These methods provide a fine, water-insoluble white powder with low solubility and notable lubricating properties.

Its primary application has been as a lubricant and flow aid in pharmaceutical tablet and capsule manufacturing. It reduces interparticle friction during compaction, eases ejection of tablets from dies, and prevents sticking to punches and machinery. When included at low levels (typically 0.5–1.0% w/w), it strikes a balance between effective lubrication and minimal negative impact on tablet integrity. It is the most commonly used lubricant in oral solid dosage forms globally; its use significantly improves powder flow and manufacturing efficiency.

Magnesium stearate also serves as an anti-adherent and release agent in confectionery and food processing. Its hydrophobic, slippery powder prevents sticking of candy to equipment and contributes to improved surface finish in pressed candies. It finds use as an anticaking agent in spices, powdered drinks, and baking ingredients where free-flowing powders are essential.

A critical application topic has been its influence on drug dissolution. Studies show that extended mixing or excess magnesium stearate can form a hydrophobic film on particle surfaces, hindering wettability and slowing drug release. Different forms and particle morphologies of magnesium stearate—anhydrous, mono-, di-, or trihydrate forms, and various polymorphs—impact functionality. Micronized grades (<20µm) are favored to ensure consistent lubrication and minimal coating effects. Careful selection of grade and blending time is essential for optimizing tablet hardness, dissolution rate, and bioavailability.

Safety and toxicological evaluations support its long-standing reputation as a low-toxicity excipient. Genotoxicity testing, including bacterial reverse mutation assays, chromosomal aberration assays in cultured mammalian cells, and in vivo micronucleus tests, have shown no evidence of genotoxic potential at levels corresponding to dietary or pharmaceutical exposure. Regulatory agencies such as the Joint FAO/WHO Expert Committee on Food Additives (JECFA) have classified magnesium stearate with an acceptable daily intake of "not specified," indicating that no toxicological concern is anticipated at current use levels.

Oral toxicity studies in rodents demonstrate that LD₅₀ values exceed 10 g/kg body weight, placing magnesium stearate in the category of practically non-toxic substances. Dietary exposure assessments estimate that children and adults may ingest tens of milligrams per kilogram per day through food additives, but this does not result in adverse health outcomes given rapid breakdown into stearic and palmitic acids and magnesium ions, all of which are common dietary components.

Emerging research continues to explore magnesium stearate’s utility in novel applications such as particle-particle dry coating, where it modifies the surface properties of carrier particles for dry powder inhalers or composites. Its role in polymer stabilization, anti-sticking agents in rubber and paper industries, and in specialized manufacturing processes such as dry particle film coating has also been documented.

In summary, magnesium stearate was discovered through exploratory studies of metal salts of fatty acids and has since become a foundational excipient in pharmaceutical and food industries. Its industrial manufacture involves stearic acid reaction with magnesium compounds to yield a fine powder with excellent lubrication and release characteristics. Widely applied in tablet and capsule production, confectionery, cleaning, and powder handling, it improves manufacturability and product consistency. While its potential to slow drug dissolution is recognized, this effect is managed through careful formulation design. Peer-reviewed toxicology and regulatory evaluations confirm its safety for human use. Its ongoing relevance underscores the success of a simple metal soap in modern industrial chemistry.

References

A review: Preparation, Characterization and Applications of Magnesium Stearate, Cobalt Stearate and Copper Stearate (peer-reviewed review article; describes widespread applications across industries including food, pharma and polymers) DOI/URL

Moreton RC (2024) Magnesium Stearate – Its Importance and Potential Impact on Dissolution of Oral Solid Dosage Forms. Dissolution Technology 31(04) 122–131 DOI: 10.14227/DT310324P122

Hobbs CA et al. (2017) Magnesium stearate a widely-used food additive exhibits a lack of in vitro and in vivo genotoxic potential. Mutation Research 821 13–20 DOI: 10.1016/j.toxrep.2017.10.003