I would be VERY suspect of anything (including "studies" b/c that "study" was likely PAID FOR by these companies that have "something to lose" should stevia gain market share) that says that Stevia is bad for you. Why? B/c they pose a serious threat to the fake sweetner companies (like Monsanto who ...btw... is behind a lot of the FDA "crack down" on Stevia which started in 1991).
Stevia is the ONLY truly natural 'no calorie" sweetener straight from a natural plant. It's been in use by the South American "Indians" (rather Natives) for an estimated 600+ years. It was "discovered" by white man 100 years ago.
Japan (who's Ministry of Health... an equivalent of our FDA (only better) and is historically FAR stricker than our biased FDA) approved this for wide use as a sweetener in 1975. Last I had heard, there had not been a single report of a problem.
In this country, the FDA tried really hard to get it banned as a sweetener and succeeded until 1994 when Senator Orin Hatch of UT introduced the dietary supplement law that was later passed. TG! B/c now it's sold this way. (Even today, no company can call it a sweetener outright. The FDA will shut them down in a heartbeat.)
There's a book ["The Stevia Cookbook" by Gates and Sahelian] that discusses the federal cover-up and the drug/chemical company involvements. It's actually a cookbook but the older version I had devoted the first 40% of the book to the background on this plant. It was very informative. Recipes... eh. They are "ok" but certainly more than good enough to get you started. (Be ware: if you buy an older print of this book, then know that the processing of stevia has gotten better so that even less is more. For example, if an older recipe for stevia calls for 1 tsp of the powder in a recipe, today that recipe might only need a 1/2 tsp, if that. I'm not sure if the 2004 update corrected these differences or not. I haven't looked at it in person.)
I have another cookbook for stevia that's better but does NOT go in to any background on it. Frankly, I'd buy the book I named just for that information. It will open your eyes in ways you've not done previously.
And btw... some say they've had headaches after using stevia. I'd say those were caused more from detoxing from the decrease or complete cessation of the use of sugar and/or chemical sweeteners. (Both of which I've used in the past and both caused me problems for a few weeks when I quit them. I have NEVER used a "fake" sweetener since the late 90's. I still use sugar for various things but not nearly as much as I used to.)
I've used stevia for 10 years, approx. Right now, I use it everyday. No problems here!
Whatever you do, avoid aspartame like your life depended on it. (It does.) I could go in to a whole host of issues with it but I don't have the time right now. I will leave you with one bit of info... aspartame though calorie free DOES cause an insulin response in the body causing you to crave even MORE sweets. Not to mention the damage it will do to your brain...
but I digress.
Anyway, HTH!