There are a number of artificial sweeteners on the market today, harmful to varying degrees. Evidence is that the worst is aspartame, also called Nutrasweet, and some other commercial names.
Aspartame in your body breaks down into aspartate, methanol and phenylalanine, all of which are toxic and neurotoxic. Aspartate in particular is an excito-toxin that over-excites your nerves, damaging and even killing nerve cells. Aspartate (and glutamate as in MSG) are natural substances which are produced in your body in small quantities to turn on your nerve cells when needed. However typical consumption of aspartame causes an excess, exciting nerves too much and in this way is neurotoxic.
Sucralose (Splenda) does not appear to be as harmful as aspartame, but nevertheless, there are many reports of harmful reactions.
It is best to avoid artificial sweeteners altogether, and to consume real sugar in moderation. In addition, one should also avoid fructose, which is more detrimental than sucrose (cane sugar).