After reviewing scientific studies, the HHS Food and Drug Administration (FDA) determined in 1981 that aspartame was safe for use in foods. In 1987, the General Accounting Office investigated the process surrounding FDA's approval of aspartame and confirmed the agency had acted properly. However, FDA has continued to review complaints alleging adverse reactions to products containing aspartame. To date, FDA has not determined any consistent pattern of symptoms that can be attributed to the use of aspartame, nor is the agency aware of any recent studies that clearly show safety problems.
Carefully controlled clinical studies show that aspartame is not an allergen. However, certain people with the genetic disease phenylketonuria (PKU), those with advanced liver disease, and pregnant women with hyperphenylalanine (high levels of phenylalanine in blood) have a problem with aspartame because they do not effectively metabolize the amino acid phenylalanine, one of aspartame's components. High levels of this amino acid in body fluids can cause brain damage. Therefore, FDA has ruled that all products containing aspartame must include a warning to phenylketonurics that the sweetener contains phenylalanine.
Source: Excerpted from FDA Consumer, May 1994 (Updated December 2004): Food Allergies Rare but Risky