Phenylalanine is an essential amino acid (a building block for proteins in the body), meaning the body needs it for health but cannot make it.  You have to get it from food or supplements.  Phenylalanine is found in 3 forms:  L-phenylalanine, the natural form found in proteins; D-phenylalanine (a mirror image of L-phenylalanine that is made in a laboratory), and DL-phenylalanine, a combination of the 2 forms.

The body changes phenylalanine into tyrosine, another amino acid that’s needed to make proteins, brain chemicals, including L-dopa, epinephrine, and norepinephrine, and thyroid hormones.  Because norepinephrine affects mood, different forms of phenylalanine have been proposed to treat depression.  Symptoms of phenylalanine deficiency include confusion, lack of energy, depression, decreased alertness, memory problems, and lack of appetite.

On the other hand, a rare metabolic disorder called phenylketonuria (PKU) occurs in people who are missing an enzyme that the body needs to use phenylalanine.  That causes high levels of phenylalanine to build up.  If it is not treated before 3 weeks of age, PKU can cause severe, irreversible intellectual disability. In the United States, newborns are tested for PKU during the first 48 – 72 hours of life.

People with PKU must eat a diet that avoids phenylalanine and take tyrosine supplements to have optimum brain development and growth.


Chronic pain – D-phenylalanine (but not L-phenylalanine) has been proposed to treat chronic pain, but so far the studies have not shown good evidence that it works.  Two studies that appeared to show positive results had flaws that made the findings unreliable.  Other clinical studies have found D-phenylalanine to be no more effective than placebo in reducing pain.  No randomized controlled trials have been done in the past 20 years to study any possible benefit of taking D-phenylalanine for chronic pain, so more research is needed to determine its safety and effectiveness.

Parkinson’s disease – One animal study suggests that D-phenylalanine may improve rigidity, walking disabilities, speech difficulties, and depression associated with Parkinson’s disease.  However, there is no evidence yet whether it would have the same effect in humans.  More research is needed.

Vitiligo – Some preliminary studies have found that L-phenylalanine (oral and topical) strengthens the effect of UVA radiation for people with vitiligo.  Vitiligo is a condition where irregular depigmentation (loss of color) causes white patches of skin.  L-phenylalanine may lead to some darkening or repigmentation of the white patches, particularly on the face.  However, more research is needed to see whether L-phenylalanine is truly effective.

Depression – Some clinical studies suggest that phenylalanine may be helpful as part of a comprehensive therapy for depression.  However, most of the studies were done in the 1970s and 1980s and were not rigorously tested.  People have reported that their mood improved when they took phenylalanine.  Researchers think this is because phenylalanine increases production of brain chemicals, such as dopamine and norepinephrine.  More research is needed to tell whether phenylalanine has any real effect on depression.

Dietary Sources

L-phenylalanine is found in most foods that contain protein such as beef, poultry, pork, fish, milk, yogurt, eggs, cheese, soy products (including soy protein isolate, soybean flour, and tofu), and certain nuts and seeds.  The artificial sweetener aspartame is also high in phenylalanine.

D-phenylalanine is made in the laboratory but is not found in food.

Available Forms

  • D-phenylalanine
  • L-phenylalanine
  • DL-phenylalanine (50/50 blend of D-phenylalanine and L-phenylalanine)

Phenylalanine can be taken as a capsule, powder, tablet, topical cream, or now in a Texas Transdermals’  time release transdermal supplement patch.