Don't worry about the phenylalanine; that warning is irrelevant to you.
Phenylalanine is an essential amino acid (ie your body absolutely requires some of it for growth and repair of muscle). There are high quantiites of it in breast milk, meat, fish, cheese and varying amounts in veg. Normally your body uses some Phe, and breaks the rest down to another amino acid (tyrosine) which the body also uses . If there is more tyrosine than the body requires at that time it excretes it.
Some people have a genetic condition called
Phenylketonuria (PKU) that means the enzyme that does the 'breaking up' of the Phe is dysfunctional. Babies with the condition are born healthy because the mothers system gets rid of the excess . After birth though, the phenylalanine, instead of converting to tyrosine, converts to another very toxic molecule, this builds up in the blood and brain causing damage. (untreated it can lead to severe brain damage)
You stop this happening by a very restrictive diet (see link for just how restrictive)
http://depts.washington.edu/pku/about/diet.html
All babies in countries with developed health care systems are tested for PKU .
I find all the diet drinks far too sweet, I can just about manage half a glass of normal diet coke (coca light here) on a very hot day but took one mouthful of coke zero and that was enough. It was horrible.
If you like it; fine but I do think that at the least it must perpetuate a liking for sweet things. The jury is out on the longterm healthiness (or not) of them
http://www.hsph.harvard.edu/nutritionsource/healthy-drinks/artificial-sweeteners/