Aspartame is very well documented as an excitotoxin, meaning that it overexcites nerve cells, causing permanent damage. According to research by Dr. Russell Blaylock, aspartame causes brain lesions, migraine headaches, reproductive disorders, Alzheimer's disease, blindness, mental confusion and many other nervous system disorders.
Part of this is probably due to the fact that aspartame, when consumed, breaks down into several chemicals, and one of those chemicals is formaldehyde. Formaldehyde, of course, is a potent nerve toxin and has no place in the human body, yet people are effectively "drinking formaldehyde" every time they pick up a diet soft drink or "diet" food item sweetened with aspartame.
In my opinion, aspartame should be immediately banned from the human food supply. It is a highly toxic ingredient that has been clearly shown to be extremely dangerous to humans. It only remains legal because the FDA wants to protect the profits of private industry rather than protect the public health. There's also the added benefit (to the drug industry) that since aspartame makes people sick, those people will need prescription drugs to mask the symptoms caused by aspartame. So, in effect, aspartame is a revenue generator for the pharmaceutical industry, and the FDA no doubt realizes this.
A much safer sweetener would be stevia, an herbal sweetener that remains illegal to use in foods in the United States even though it is used around the world by hundreds of millions of people each year with no negative side effects. The FDA refuses to recognize stevia as a safe food ingredient in the United States, so you can only purchase it as a nutritional supplement. Make no mistake: the FDA will resist legalizing stevia for as along as possible, making sure the profits for aspartame are well protected, regardless of the cost to human health.