Are artificial sweeteners safe?
(the less-interested can just read the boldface parts)

1. Vocabulary
        aspartame == nutrasweet == Equal
        saccharin == Sweet 'n' Low, Sugar Twin
        sugar alcohols == mannitol, sorbitol, xylitol, et al.

2. The famous internet Nutrasweet documents are not to be trusted.

        This backlash against aspartame follows the common quack MO of attributing fatigue, headaches, and other "problems of life" to a single, poorly understood cause. There are some people, phenylketoneurics by name, who become severely ill if they consume aspartame. It is possible that some of these reported reactions are due to sub-clinical phenylketonurea.
        The issue of toxic product breakdown is evidently a live one. It appears uncontested that aspartame is metabolized into methanol, the toxicity of which I need not belabour. One scholarly-seeming report on the internet suggests that even very high aspartame consumption won't trigger acute toxicity, but may exceed the EPA's allowable intake to avoid cumulative effects. Limiting consumption should eliminate this problem - humans do have some limited ability to process methanol safely, as it is present naturally in many foods (I find the author's suggestion that it is partially neutralized by the ethanol in these foods unreasonable; ethanol prevents methanol toxicity by competing for metabolic enzymes, which should not occur with such minuscule doses).
        Acute toxicity may be a problem if aspartame is allowed to break down prior to consumption. This MAY occur if it is heated (which is why Equal is not to be used in cooking - unfortunately, it also tastes awful when heated excessively, which has prevented us from gathering data from those foolish enough to ignore the warning). I have not been able to verify/debunk the belief that diet soda is shipped under constant refrigeration, but to be safe I would avoid drinking aspartame-sweetened beverages that have been stored above 80F for any significant period of time, or eating aspartame-sweetened foods that have undergone significant cooking (I don't know whether the foul taste and the toxic products are inseparable, or if one precedes the other).
        Another camp claims that aspartame causes neurotoxic damage (such as MS) by overexciting NMDA receptors. It is true that aspartic acid, a related substance, is an excitatory neurotransmitter (glutamic acid, a metabolite of MSG, has similar effects). However, the research on this topic is so idieologically charged that I do not consider my cursory examination sufficient to evaluate it. It scares me enough that I try to limit my consumption of aspartame (and MSG as well), but I'm far from convinced.
        Verdict: Limit consumption of aspartame. In general, do not consume any substance that reliably causes headaches, nausea, or other disagreeable symptoms without a great deal of information.

3. Saccharin may be okay.
        The 1970s-era findings by U.S. regulatory agencies that saccharin may cause cancer have been brought into question by modern animal and epidemiological studies. Initial studies were carried out on rats, which appear to be uniquely susceptable to the sodium ion in commonly-used forms of saccharin. Please see saccharin for a more detailed discussion of the scientific literature.

4. Stevia is not as bad as they say.
        Stevia is a plant-derived sweetener which is bitter in large quantities but tastes rather like sugar in minute ones. The FDA is waging a holy war against it, with little enough evidence that I am entirely willing to believe those who claim that it's all part of a conspiracy orchestrated by the makers of aspartame and saccharin. Supporters claim that stevia is 100% safe, which is untrue for any substance. However, it is widely used throughout Asia, indicating that it lacks at least acute toxicity.
        Stevia may not be sold in the U.S. as a sweetener, but is available as a "dietary supplement," meaning that the producer and the consumer both have to pretend they don't know what it tastes like or how it's to be used. It's available in natural health stores. Stevia is probably not acutely dangerous, but it hasn't been subjected to the same rigorous testing as other approved artificial sweeteners.

5. But this may all be irrelevant.
        Safety is one thing. Efficacy is another. And no, I'm not talking about the fact that diet soda tastes like something I'd want a fume hood for (this coming from someone who used to drink three Diet Pepsis™ a day, minimum). But people generally use artificial sweeteners to reduce their consumption of sugar and other caloric foods. Does this work? In an elegantly designed experiment, Rogers and Blundell (Physiology and Behavior, 1989. 45. 1093-1099) fed subjects yogurt that varied in caloric content and caloric vs. non-caloric sweetener. They found that both hunger and food intake during a post-yogurt lunch and throughout the day were increased in subjects given saccharin, such that they ended up eating significantly more than all other groups by the end of the day. This result has been replicated and is on solid theoretical grounding (the cephalic phase response, by which sweet tastes evoke insulin, lowering blood sugar and creating a carbohydrate craving, is well-known), but some long-term results don't show any consequent weight gain from free-living or institutionalized humans placed on high-sweetener diets (Rolls. American Journal of Clinical Nutrition, 1991. 53. 872-878.). Therefore, there may be adaptation to artificial sweeteners, or simple methodological confounds that have yet to be unravelled. However, it would prudent to exercise some oversight - people have been known to engage in the most appalling abuses of "free" foods (we all know someone who drinks 12 diet sodas a day). If artificial sweeteners just leave you craving more (or perhaps craving real sugar), it may be worthwhile to take a trial period without them and see whether your caloric intake actually changes.

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