Aspartame is the artificial sweetener in Diet Coke. It is a $570M per year product. It is said to have caused Gulf War Syndrome, contributes to multiple sclerosis, can cause petit mal seizures, and is transformed in the body to deposit formaldehyde in the brain.
The president consumes dozens of diet cokes a day, according to a 12/10/17 NYT article. This could be a contributing factor to his erratic behavior. In any case, I would counsel against consuming in such large amounts.