I read that Phenalalanine is not hamful but i dont believe it. Are they lying?

When I see "Phenylketoneurics: Contains Phenylalanine" on the side of a diet soda can, I think that those are some pretty big words to be ingested. I looked it up and read that it is not hamful at all unless you were born with Phenylketonourics (PKN) which you are tested for at birth. I dont buy it. Anyone have any other information?