Stevia has been found to be a weak mutagen in multiple studies. Many of those aspartame studies were funded by the sugar industry feeding rats dosages many times what a human would be capable of consuming, distilled water will kill you if consumed in a large enough quantity. Years ago, the head of the FDA claimed table sugar wouldn't be approved by the FDA as a food additive if it went through the approval process today.
http://www.aspartame.org/
http://www.aspartame.org/aspartame_myths.html
Aspartame is the methyl ester of the dipeptide of the natural amino acids L-aspartic acid and L-phenylalanine. Under strongly acidic or alkaline conditions, aspartame first generates methanol by hydrolysis. Under more severe conditions, the peptide bonds are also hydrolyzed, resulting in the free amino acids. It is a nonpolar molecule.[1]
Upon ingestion, aspartame breaks down into several residual chemicals, including aspartic acid, phenylalanine, methanol, and further breakdown products including formaldehyde[2] and formic acid. There is some controversy surrounding the rate of breakdown into these various products and the effects that they have on those that consume aspartame-sweetened foods.
The naturally-occurring essential amino acid phenylalanine is a health hazard to those born with phenylketonuria (PKU), a rare inherited disease that prevents the essential amino acid phenylalanine from being properly converted into Tyrosine and eventually being metabolized. Since individuals with PKU must consider aspartame as an additional source of phenylalanine, aspartame-containing foods sold in the United States must state "Phenylketonurics: Contains Phenylalanine" on their product labels.
What I find ironic is that many people who refuse to drink a diet coke because of aspartame have no problem throwing back twelve alcoholic drinks in an evening.
1.David J. Ager, David P. Pantaleone, Scott A. Henderson, Alan R. Katritzky, Indra Prakash, D. Eric Walters (1998). "Commercial, Synthetic Nonnutritive Sweeteners". Angewandte Chemie International Edition 37 (13-24): 1802-1817. DOI:10.1002/(SICI)1521-3773(19980803)37:13/14%3C1802::AID-ANIE1802%3E3.0.CO;2-9
2.Trocho, C.; Pardo, R.; Rafecas, I.; Virgili, J.; Remesar, X.; Fernandez-Lopez, J.A.; Alemany, M. Formaldehyde derived from dietary aspartame binds to tissue components in vivo., Life Sci., 1998, 63(5), 337-349