Magnesium stearate is a FDA-approved excipient widely used in food, supplements and pharmaceutical products; to act as a flow agent or lubricant to prevent ingredients from sticking to the machine that creates the capsules or tablets.
It is a made using stearic acid and magnesium, and manufacturers typically only use small amounts of magnesium stearate in products. According to the National Center for Biotechnology Information, it is considered safe for consumption at amounts below 2,500mg per kg body weight daily. Thus, the products don't contain enough magnesium stearate to cause negative side effects. (Val: don't understand)