L-phenylalanine is an essential amino acid, a "building block" of protein. Major dietary sources of L-phenylalanine include meat, fish, eggs, cheese, and milk.

L-phenylalanine can be converted to L-tyrosine and subsequently to L-dopa, norepinephrine, and epinephrine. L-phenylalanine can also be converted (through a separate pathway) to phenylethylamine, a substance that occurs naturally in the brain and appears to elevate mood.