Aspartame is the methyl ester of the dipeptide of the natural amino acids L-aspartic acid and L-phenylalanine. This is why early marketing efforts described it as made from natural amino acids. Aspartame (also marketed as Equal and NutraSweet) is 180 times sweeter than common white table sugar. After you ingest it in your body, aspartame breaks down in various chemicals These chemicals include aspartic acid, phenylalanine, methanol, and further breakdown products
including formaldehyde, formic acid, and a diketopiperazine.

Did you see formaldehyde listed above? That can't be good. In fact, aspartame was criticized in the 1980's by various groups as a suspected cause of brain cancer. 

While many have heard that aspartame causes everything from headaches to seizures, and most health food stores avoid products sweetened with aspartame, aspartame is composed of two amino acids found in any health food store, Aspartic Acid and Phenylalanine only these are altered by man.. There is a condition known as phenylketonuria (PKU), a congenital condition that makes it impossible for the body to metabolize phenylalanine. If an individual with PKU ingests aspartame, or pure phenylalanine for that matter, they do run serious risks. Toxic compounds accumulate in the body and lead to nerve damage and, in some cases, severe brain damage.