Does aspartame cause cancer or tumor formation?

No. Long-term and lifetime tests in rats and mice with extremely large amounts of aspartame showed no evidence of brain tumors or any kind of cancer associated with aspartame.

In fact, it is physiologically impossible for aspartame to cause cancer. Aspartame itself never enters the bloodstream, rather, it is broken down in the gastrointestinal tract to small amounts of common dietary components including the building blocks of protein, aspartic acid and phenylalanine, and methanol. We consume these same components in much greater amounts in common foods, such as milk, meat, fruits and vegetables. The body handles these components from aspartame in the same way it handles them from other food sources.