L-phenylalanine—an essential amino acid your body is not able to produce naturally on its own—may help your body deal with daily stress and help promote a positive mood. This is the number one reason why people choose to use L-phenylalanine supplements. It’s a building block for various proteins that your body produces. L-phenylalanine (LPA) can ultimately be converted into L-dopa, norepinephrine and epinephrine by way of first being turned into another amino acid, L-tyrosine. LPA may also convert into phenylethylamine, nutrient that works inside your brain and appears to promote a positive mental outlook.