DL-Phenylalanine is a combination of the well-tolerated essential amino acids, d-phenylalanine and l-phenylalanine. Together, these amino acids have been studied for their potential to promote mental functioning and to support positive mood.1 l-Phenylalanine is a precursor to l-tyrosine, which is converted to the catecholamine neurotransmitters norepinephrine, epinephrine, dopamine and tyramine.2 l-Tyrosine and its metabolites play a role in emotional well-being, memory and learning.3 d-Phenylalanine may promote the levels of certain endorphins, which play a role in maintaining muscle and joint comfort.4‡