There are some questions re: adverse effects on Stevia, although inconclusive (also some positive benefits as discussed below);
http://en.wikipedia.org/wiki/Stevia#Health_controversy"Health controversy
A 1985 study reported that steviol, a breakdown product from stevioside and rebaudioside (two of the sweet steviol glycosides in the stevia leaf), is a mutagen in the presence of a liver extract of pre-treated rats[20] — but this finding has been criticized on procedural grounds that the data were mishandled in such a way that even distilled water would appear mutagenic.[21] More recent animal tests have shown mixed results in terms of toxicology and adverse effects of stevia extract, with some tests finding steviol to be a weak mutagen[22] while newer studies find no safety issues.[23][24]
Other studies have shown stevia improves insulin sensitivity in rats[25] and may even promote additional insulin production,[26] helping to reverse diabetes and metabolic syndrome.[27] Preliminary human studies show stevia can help reduce hypertension[28] although another study has shown it has no effect on hypertension.[29] Despite these more recent studies establishing the safety of stevia, government agencies have expressed concerns over toxicity, citing a lack of sufficient conclusive research.[30][31]"
