Stevioside and rebaudioside A are diterpene glycosides, composed of the plant product steviol, with 3 or 4 glucose molecules attaced.
The Coca-Cola Company and Cargill have undertaken or sponsored a wide array of safety studies in conjunction with their development of rebaudioside A as a commercial sweetener. Eleven of these were published in Food and Chemical Toxicology and are available in the on-line edition.
Bacteria in the colon remove the sugars from stevioside and rebaudioside, and the remaining steviol molecule is absorbed by the body. It is conjugated in the liver (glucuronic acid is attached to make it more water-soluble) and then excreted in the urine.