Does phenylalanine cause cancer?

No: Phenylalanine is an essential Amino Acid required for healthy protein metabolism.

Likely low risk: This is a common artificial sweetener. The data is controversial. In animal models, you can observe a slightly higher cancer risk if you feed huge amounts of this chemical to rodents. How that translates to humans who ingest smaller amounts over the course of years is not totally known. As with everything, try to moderate your intake.