About fifty per cent of aspartame is made up by phenylalanine, which could cross the blood-brain barrier and act as a precursor of catecholamine in the brain, as well as leading to phenylketonuria.
Forty per cent of aspartame is composed of aspartic acid, an excitotoxin that contributes to free radical damage in the brain.
Methanol which forms 10% of the by-products of aspartame is a toxic compound converted in the liver to formaldehyde that is neurotoxic and carcinogenic.