Stevia leaf extract is Generally Recognized As Safe (GRAS) in accordance with the US FDA requirements. Crude stevia, or whole-leaf stevia, does not have GRAS status from the US FDA and has not been permitted for use as a food additive.
One way the US FDA accomplishes its food safety mission is to distinguish between “food additives” and “dietary supplements.” Food additives (such as stevia leaf extract) can be approved by the US FDA or declared to be GRAS. Food additives are described as any substance intentionally added to a food. Because stevia leaf extract has GRAS status, it is able to be sold commercially in stevia-based sweeteners available for purchase at retail in the US.
Dietary supplements are products taken by mouth that contain a dietary ingredient. Crude stevia or whole-leaf stevia—different from the highly purified forms of stevia leaf extract—are currently not permitted for use as sweeteners as per the US FDA and do not have GRAS status.
Read the full US FDA opinion here.