There are over 90 potential side effects related to phenylalanine, I can not find a complete list of that, the most complete list I found (listing about 2 dozen) do not include hair loss as one of them. However tooth whitening gels contain other chemicals which can have side effects on a sensitive person. If you are seriously anxious about this then take the product and its packaging and go to the doctor (make a same day appointment) and he/she can check against the list of chemicals in there to tell you if it could be the cause of sudden hair loss. However what is most likely is not just one product has caused this, it could be a chemical reaction of several products you use, like hair gel and tooth jell and deodorant - each places chemicals on your skin which is absorbed and combines inside of you to have an effect. It is not too uncommon for people to suffer a form of toxic shock from the use of several innocent products which in combination with their body chemistry can have nasty side effects. (Allergies develop in many cases as a form of toxic shock where the body has absorbed or has been exposed to too many chemicals one one kind or another)

Nope. Phenylalanine is perfectly safe for people who aren't phenylketonuric (a condition where the body can't metabolize the amino acid phenylalanine).