Magnesium stearate, also called octadecanoic acid, magnesium salt, is a white substance which is solid at room temperature. It has the chemical formula C36H70MgO4. It is a salt containing two equivalents of stearate (the anion of stearic acid) and one magnesium cation (Mg2+). Magnesium stearate melts at about 88 °C, is not soluble in water, and is generally considered safe for human consumption. Because it is widely regarded as harmless, it is often used as a filling agent in the manufacture of medical tablets and capsules. In this regard, the substance is also useful because it has lubricating properties, preventing ingredients from sticking to manufacturing equipment during the compression of chemical powders into solid tablets. It is used to bind sugar in hard candies. It is also a common ingredient in baby powders. In pure powder form, the substance can be a dust explosion hazard, although this issue is effectively insignificant beyond the manufacturing plants using it.
When used as a filling agent in the manufacture of capsules and tablets, such as vitamins, the source of this ingredient is typically beef. However, there is an increasing number of vegetarian options in which the product specifically indicates it contains magnesium stearate from vegetable sources.
Magnesium stearate is a major component of "bathtub rings".