Stevia has had a bit of a mixed story in the U.S. This plant has been used for centuries in South American as a sweetener and is a common sweetener in places such as Japan. In the U.S., however, stevia was banned from use as a food additive because of research that suggested stevia could potentially cause cancer.
The FDA's "non objection" results from data submitted by the industry that shows the safety of a stevia refining process that removes the suspect components of stevia. This process focuses on a component of stevia known as steviol glycoside rebaudioside A (Reb-A). Reb-A (also called Rebiana), has a different safety profile than stevia in general. Truvia is made from Reb-A and appears in products such as Sprite Green.
But is Truvia safe? Well, the FDA doesn't object and it is likely as safe as any of the sugar substitutes out there - one could argue it is less safe because of the cancer finding in the past, but one could also argue that the "naturalness" of Truvia combined with the centuries of use (as stevia) in the world make is safer. Of course, you'll have to decide for yourself.
Read more on Stevia