L-phenylalanine (LPA) serves as a building block for the various proteins that are produced in the body. 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.

D-phenylalanine (DPA) is not normally found in the body and cannot be converted to L-tyrosine, L-dopa, or norepinephrine. As a result, DPA is converted primarily to phenylethylamine (a potential mood elevator). DPA also appears to influence certain chemicals in the brain that relate to pain sensation.

DLPA is a mixture of the essential amino acid LPA and its mirror image DPA. LPA is found in most foods that contain protein. DPA does not normally occur in food, but when synthesized in the laboratory, half appears as LPA and half as DPA. The combination supplement (DLPA) is often used because both components exert different health-enhancing effects.

Although rare, individuals whose diets are very low in protein may develop a deficiency of L-phenylalanine. Benefits of supplementation are typically achieved in the absence of an outright deficiency.

DLPA has been used in amounts ranging from 75 to 1,500mg per day. Consistent toxicity in healthy people has not been reported with 1,500mg per day or less of DLPA, except for occasional nausea, heartburn, or transient headaches.
This compound can have powerful effects on mood and on the nervous system, and therefore DLPA should be taken only under medical supervision.

People with phenylketonuria must not supplement phenylalanine. Some research suggests that tardive dyskinesia patients may process phenylalanine abnormally. Until more is known, it makes sense for people with this condition to avoid phenylalanine supplementation.

Since DLPA competes with other amino acids for attachment on a common amino acid carrier in the body, it should not be taken with protein containing foods. Individuals taking prescription or over-the-counter medications should consult a physician before taking DLPA.