There are few risks from taking supplements that contain magnesium stearate, according to Toxmet. It is not carcinogenic and is just slightly toxic if ingested. Any deaths from magnesium stearate have been caused by accidental inhalation of a powder that contains it.
Magnesium stearate is an inactive substance that is used to dilute other ingredients in a capsule, tablet or powder, says Drugs.com. It has lubricating properties that prevents ingredients in medications or supplements from sticking to laboratory equipment. It is also called octadecanoic or stearic acid and has a chemical formula of Mg(C18H35O2)2. Magnesium stearate is a saturated fatty acid with a backbone made of 18 carbon atoms, says PubChem.
Medications that commonly have magnesium stearate include acetominophen and hydrocodone bitartrate, alprazolam, cyclobenzaprine hydrochloride, hydromorphone hydrochloride, methocarbamol and morphine sulfate, says Drugs.com. Other medications that use magnesium stearate are promethazine hydrochloride, tramadol hydrochloride and Xanax.
Magnesium stearate is derived from both plants and animals. It's found in cocoa butter, tallow and vegetable fats, says HowStuffWorks. It is also a surfactant, which is a substance that reduces the surface tension of water. This allows water to attach to oils on a surface and take them away. It also binds materials that would normally repel each other, such as oil and water.