The amino acid Phenylalanine exists in two forms, the D- and L- forms. L-Phenylalanine(LPA, C9H12NO2) is a neutral amino acid found in proteins, coded for by DNA. Its mirror image, D-phenylalanine (DPA), can be synthesized artificially.

L-phenylalanine is used in the human body, where it is an essential amino acid. L-Phenylalanine can also be converted into tyrosine. Tyrosine is converted into L-dopa, norepinephrine, and epinephrine. D-phenylalanine can only be converted into phenylethylamine.

The genetic disorder phenylketonuria is an inability to metabolize phenylalanine.

The synthesized mix DL-Phenylalanine (DLPA), which is a combination of the D- and L- forms, is used as a nutritional supplement.