There are no verified health risks associated with the aspartame in diet drinks, as of 2015, other than for people who suffer from a condition known as phenylketonuria. The Food and Drug Administration has ruled aspartame safe, as has the European Food Safety Administration
There is no credible scientific evidence to back up claims that aspartame is related to conditions such as digestive problems or dizziness, nor to more serious illnesses such as diabetes, birth defects, seizures or Parkinson's disease. Some people have a rare genetic condition called phenylketonuria, in which their bodies cannot break down phenylalanine. People suffering with this condition should not consume aspartame, as phenylalanine is an ingredient.
Researchers testing on both animals and people have studied the link between aspartame and cancer, and no research has conclusively found a causal relationship. Research that claimed otherwise is questionable based on poor data or results that could have occurred by chance, according to Cancer.org. The Food and Drug Administration sets acceptable daily limits for sweeteners such as aspartame that are 100 times less than the level that has been shown to cause harm. They set the daily limit for aspartame at 50 milligrams per kilogram of a person's body weight, the equivalent of drinking more than 19 cans of diet drinks per day.