The safety issue first arose in the '60s, when cyclamates were banned as likely carcinogens. Saccharin, which is found in Sweet'n Low, was labeled a possible carcinogen in the early '70s and had to carry a warning until 2000. Products with aspartame (NutraSweet) have always been labeled dangerous for people with phenylketonuria, a relatively rare condition. And sucralose (marketed as Splenda) bears no warning label at all.
How should consumers interpret all this? Michael Jacobson, director of the Center for Science in the Public Interest, believes there's no "slam dunk" proof that any of the artificial sweeteners is clearly dangerous or perfectly safe. But "based on what we know so far," he says, "I think that sucralose is safe, that aspartame is probably safe and that serious questions about saccharin remain"--though the risk to an individual is very low.
Some critics also claim that artificial sweeteners can actually make you fatter by turbocharging your sweet tooth, but that's folk wisdom with no hard evidence. The real question is whether they do any good. For losing weight, diet soda is better than regular, but water is even better. And pastries made with artificial sweeteners can still be high in calories if they're high in fat. If you get a Big Mac and fries with that Diet Coke, you're simply going to get fat at a slightly slower rate. Fake sugar is probably not a terrible thing but it's not so great for you either.