This is usually added as a lubricant to prevent tablet and capsule contents from sticking to the machinery that processes them. Stearic acid is an organic compound that is splits from magnesium during digestion. To date, we are not aware of any negative complete research. We do know that the source of the magnesium stearate is important - some can be made from hydrogenated oils. If a manufacturer tests for purity and it's backed by USP (http://www.usp.org/usp-verification-services/usp-verified-dietary-supplements) then we believe it should be fine. However, supplements can be made without them – it just takes more time, care and attention to detail. So if you can find one w/o then I would choose that one since we know for sure it's pure.