An essential amino acid. L-Phenylalanine is also the precursor to l-tyrosine as well as phenethylamine (PEA). It may be included in certain weight loss/bodybuilding supplements for this reason, as both metabolites have mood/cognitive effects.
Phenylalanine is a product of aspartame metabolism. Various activists insist it’s (partly) responsible for the reputed (and often exaggerated) side effects of the sweetener, since it’s a “neurotoxin” when taken in isolation. This claim is based on studies of phenylketonuria (PKU), a genetic disorder that affects phenylalanine metabolism. However, there is no reason to believe that small amounts of phenylalanine, taken in supplemental form, will have any adverse effects on healthy people.