There is a positive correlation with cancer, but all that testing isn't complete yet. What really bugs me about aspartame is that it appeals to people who diet. So they drink lots and lots of it. There have been many instances where people who drink aspartame in drinks develop the same exact symptoms as people with MS. Someone in my area had these symptoms and they were getting worse and worse. When they finally got to see a specialist, the first thing they did, even before any testing, was to ask the parents to remove aspartame products from the home for two weeks and guarantee that the patient not have any aspartame during that time. Within ONE WEEK all the symptoms were completely gone. I've heard other stories like that. I know an oncologist (cancer doctor) who would never drink aspartame. He doesn't have it in his house. The studies are not yet finished, but he says that everything points in that direction.
There are conflicting accounts of whether or not it has adverse effects. The cancer society says there is only a link in laboratory tests .. and that's because there are such high quantities used in the testing.. Here is a link of someone who thinks otherwise .. http://www.sweetpoison.com/aspartame-information.html