Phenylalanine supplementation helps in the production of neurotransmitters (dopamine, norepinephrine and epinephrine). Clinical trials suggest it may promote positive moods. Phenylalanine is an essential amino acid, which means your body can’t produce sufficient amounts and requires it from an external source. It’s converted to L-tyrosine in the liver, which is then converted into dopamine, norepinephrine and epinephrine (adrenaline) hormones that can become depleted for a variety of reasons. Replenishing norepinephrine in the brain can support brain health.