Phenylketonurics: Contains Phenylalanine.
Why should I care??
You would have found out about this when you were born. If you have that condition, which I’m too lazy to retype, and you consume products with phenyalanine, you could develop mental retardation and a lot of other nasty stuff.
The labelling is an FDA requirement.
With keywords like phenylketonurics and phenylalanine, this was an easy one to find in the archives.
http://boards.straightdope.com/sdmb/showthread.php?threadid=29212
There are other threads, but I’m too lazy to copy them all. This one seems to cover most of the facts.