Aspartame is the food additive E951 (E-numbers are a certificate of safety in human consumption from the European Union).
It remains in the system for a short time, doesn’t accumulate, and is broken down into 50% phenylalanine, 40% aspartic acid, and 10% methanol which are substrates naturally found in common foods (tomato, milk, egg, and meat).

 Even while it is briefly in the body as aspartame it is only toxic at incredibly high doses. The ADI is 40-50mg/kg according to ESFA, WHO, FDA, JECFA, SCF, ANS, and 90 other organisations around the globe. This ADI is calculated by taking the lowest dosage at which side effects are observed before multiplying by a factor of 100.

All supposed “aspartame sensitive” individuals have shown to be suffering from nocebo rather than physical effects from the chemical itself.