The amino acid Phenylalanine exists in two forms, the D- and L- forms. It has a phenyl side chain. L-Phenylalanine(LPA, C₉H₁₂NO₂) is an electrically 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.

Phenylalanine is part of the composition of aspartame, a common sweetener found in prepared foods (particularly soft drinks, and gum). Due to phenylketonuria, products containing aspartame usually have a warning label that they contain phenylalanine, in compliance with U.S. FDA guidelines.

The genetic codon for phenylanaline was the first to be discovered. Marshall W. Nirenberg discovered that when he inserted RNA made up of multiple uracil repeats into E. coli, the bacterium produced a new protein, made up solely of repeated phenylalanine amino acids.

Phenylalanine uses the same active transport channel as tryptophan to cross the blood brain barrier, and in large quantities interferes with the production of serotonin.