In spite of chat rooms dedicated to blaming diet soda for every conceivable health problem and the fall of American values, no scientific study to date has shown ANY negative side effect of aspartame even at the upper bounds of current human consumption.

And in spite of those studies, I get a terrible splitting headache within minutes of drinking a diet soda containing aspartame.

I'm in the middle of preparing a proposal that explains one way in which all previous aspartame studies are flawed. Sorry, not going to explain it now. Aspartame studies are actually pretty complicated. One flaw, which is not the flaw I'm focusing on, is that studies are done using fresh aspartame, even though it's known that aspartame breaks down into other by-products after sitting on a shelf for a few months. Those by-products are not studied for safety.

The vast majority of studies demonstrating the safety of aspartame were done in lab animals who are incapable of getting, or reporting if they did get, many of the symptoms that people claim to get from aspartame.

Another interesting fact about aspartame is that many of the studies demonstrating its safety were funded by Donald Rumsfeld, who was CEO of Searle at the time.

There's some information here about the history of its approval. I don't know whether it's accurate.