Because it is widely regarded as harmless in small doses, it is often used as a filling agent in the manufacture of medical tablets and capsules. However, problems of toxicity, absorbtion and immunosupression are noted below.http://en.wikipedia.org/wiki/Magnesium_stearate

Physical hazards: Explosive hazard when concentrated dust is present.
Health hazards: Irritant

May cause irritation with symptoms of redness, swelling, itching and pain.
Eye contact may cause irritation with symptoms of redness,s welling, itching, tearing and pain. May cause gastroenteritis (inflammation of the lining membrane of the stomach and intestines) with abdominal pain, nausea, vomiting, and diarrhea. May cause irritation to the respiratory tract. The product should be prevented from entering drains, sewers, streams, etc. Toxicological: liver – L70 nutritional and gross metabolic – weight loss or decrease in weight gain.

Companies that manufacture and transport magnesium stearate must file a Material Safety Data Sheet with the Environmental Protection Agency because concentrated magnesium stearate is classified as a hazardous substance. Its uses are listed as “ammunition, dusting powder, paint and varnish drier, binder, and emulsifier.”

Beware of Additives in Supplements
Stearates – Hydrogenated Fats Used in the Production of Most Supplements– Decrease Absorption and May Be Toxic and Immunosuppressive, by Ron Schmid, N.D

Magnesium stearate, stearic acid and ascorbyl palmitate, are made by hydrogenating cottonseed or palm oil, Cottonseed oil has the highest content of pesticide residues of all commercial oils; cotton crops are heavily sprayed. In the hydrogenation process, the oil is subjected to high heat and pressure in the presence of a metal catalyst for several hours, creating a hydrogenated saturated fat. Hydrogenated vegetable fats contain altered molecules derived from fatty acids that may be toxic. The metal catalyst used in the hydrogenation process may also contaminate the stearates produced

This delays the absorption of nutrients. Individuals with impaired digestion may have particular difficulty absorbing nutrients coated with stearates.

Another problem with stearates are concentrated doses of stearic acid suppress the action of T-cells, a key component of the immune system.
Tebbey PW, Buttke TM Immunology. 1990 Jul;70(3):379-84.

Possibly adding to this immunosuppression, stearates can jeopardize utilization of essential nutrients by the individual.

No information is available on safety of parenteral (injection directly into the body) administration of magnesuium stearate.

octadecanoic acid, magnesium salt.