The overwhelming body of scientific evidence clearly demonstrates that aspartame, even in amounts many times what people typically consume, is safe and not associated with adverse health effects. However, over the years, some consumers have reported symptoms, which they believed were associated with aspartame. The FDA has investigated these allegations and concluded that there is no "reasonable evidence of possible public health harm" and "no consistent or unique patterns of symptoms reported with respect to aspartame that can be causally linked to its use." In 1984, the Centers for Disease Control (CDC) reviewed 517 of these anecdotal reports and stated, "the majority of frequently reported symptoms were mild and are symptoms that are common in the general populace" and that “focused” clinical studies would be the best way to evaluate these complaints.

see.. Diet Coke is not deadly :).