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Table of Contents > Supplements > Phenylalanine
Also Known As:  D-phenylalanine, L-Phenylalanine, DL-phenylalanine
Dietary Sources
Available Forms
How to Take It
Possible Interactions
Supporting Research


Phenylalanine is an essential amino acid, which means that it is essential to human health but cannot be manufactured by the body. For this reason, phenylalanine must be obtained from food. Amino acids are the building blocks of protein. It is available in three chemical forms: L-phenylalanine, the natural form of phenylalanine found in proteins throughout the body; (2) D-phenylalanine, a mirror image of L-phenylalanine that is synthesized in a laboratory; and (3) DL-phenylalanine, a combination of the previous two forms.

The body converts phenylalanine into tyrosine, another amino acid essential for making proteins, certain brain chemicals, and thyroid hormones. Symptoms of phenylalanine deficiency include confusion, lack of energy, decreased alertness, decreased memory, and diminished appetite.

On the other hand, a rare metabolic disorder called phenylketonuria (PKU) occurs in people who are missing an enzyme that is needed to properly metabolize phenylalanine. Symptoms of PKU, which tend to appear between three and six months of age, include eczema, developmental delay, an abnormally small head, and hyperactivity. If it is not treated before three weeks of age, PKU can cause severe, irreversible mental retardation. In the United States, newborns are tested for PKU during the first 48 to 72 hours of life.

People with PKU must eat a phenylalanine-restricted, tyrosine-supplemented diet to have optimum brain development and growth. Rarely, over-restriction of phenylalanine in the diet can lead to deficiency of this amino acid, with the same symptoms described above.


Chronic Pain
Although results of studies have not been entirely consistent, preliminary evidence suggests that D-phenylalanine may help reduce chronic pain associated with certain health conditions by stimulating nerve pathways in the brain that control pain. Some scientists, for example, report that they have observed enhanced pain relief when D-phenylalanine is used together with prescription pain-killers. Other studies have found D-phenylalanine to be no more effective than placebo in reducing pain. Further research is needed to determine the safety and effectiveness of this amino acid for pain.

Parkinson's Disease
One animal study suggests that D-phenylalanine may improve rigidity, walking disabilities, speech difficulties, and depression associated with Parkinson's disease. It is not clear whether these results translate into a possible treatment for people with this disease, however. Further studies in people are necessary before supplementation with this amino acid can be recommended for individuals with Parkinson's disease.

Evidence suggests that combining L-phenylalanine (oral and topical) with UVA radiation for people with vitiligo (a condition characterized by irregular depigmentation or white patches of skin) may lead to some darkening or repigmentation of the whitened areas, particularly on the face. Although preliminary information suggests that it is safe when used under appropriate medical guidance and supervision, more research is needed to assess potential side effects of this treatment approach.

Although far from proven, there is at least theoretic benefit in using phenylalanine as part of a comprehensive therapy for depression. In support of this theory, some individuals have reported improvement in mood when taking phenylalanine. This is thought to be due to enhanced production of brain chemicals such as dopamine and norepinephrine. More research is needed.

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 synthesized in the laboratory 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, or tablet or used as a topical cream.

How to Take It

Recommended dosages of phenylalanine vary depending on the health condition being treated. Supplements are recommended 15 to 30 minutes before meals.

According to the National Research Council, the daily recommended dietary allowances (RDAs) for phenylalanine are as follows:


  • Birth to 4 months: 125 mg per kilogram of body weight per day
  • Children 5 months to 2 years: 69 mg per kilogram of body weight per day
  • Children 3 to 12 years: 22 mg per kilogram of body weight per day

The combination of oral and topical phenylalanine (together with ultraviolet light) have been used to treat children with vitiligo. The dose and appropriate length of time to continue the therapy would be determined by your physician.


  • Teenagers and adults: 14 mg per kilogram of body weight per day

Some experts suggest that adults may need as much as 39 mg per kilogram of body weight per day for general health and doses as high as 50 to 100 mg per kilogram have been used in studies of those with vitiligo. The most common amounts used range from 750 to 3,000 mg per day for adults.


Because of the potential for side effects and interactions with medications, dietary supplements should be taken only under the supervision of a knowledgeable healthcare provider.

People with PKU and women who are lactating or are pregnant should not take phenylalanine supplements.

Little is known about the use of aspartame during pregnancy. Talk with your health care provider about using this artificial sweetener.

DL-phenylalanine may cause symptoms of anxiety, jitteriness, and hyperactivity in children. Children with ADHD may need to avoid foods containing aspartame, which is made from phenylalanine.

Doses in excess of 5,000 mg a day may be toxic and can cause nerve damage. High quantities of DL-phenylalanine may cause mild side effects such as nausea, heartburn, and headaches.

Possible Interactions

If you are currently being treated with any of the following medications, you should not use phenylalanine without first talking to your healthcare provider.

Antidepressant Medications, Monoamine Oxidase Inhibitors (MAOIs)
Phenylalanine may cause a severe increase in blood pressure in people taking MAOIs (such as phenelzine, tranylcypromine, pargyline, and selegiline). This severe increase in blood pressure (also called "hypertensive crisis") can lead to a heart attack or stroke. For this reason, individuals taking MAOIs should avoid foods and supplements containing phenylalanine.

The absorption of baclofen, a medication used to relieve muscle spasms, may be reduced by phenylalanine. Therefore, it is best to avoid taking this medication with a meal, especially one that is high in protein content, or with phenylalanine supplements.

A few case reports suggest that phenylalanine may reduce the effectiveness of levodopa, a medication used to treat Parkinson's disease. Some researchers speculate that phenylalanine may interfere with the absorption of this medication. Therefore, phenylalanine should not be taken at the same time as levodopa.

Supporting Research

Antoniou C, Katsambas A. Guidelines for the treatment of vitiligo. Drugs. 1992;43(4):490-498.

Bugard P, Bremer HJ, Buhrdel P, et al. Rationale for the German recommendations for phenylalanine level control in phenylketonuria 1997. Eur J Pediatr. 1999;158:46-54.

Burkhart CG, Burkhart CN. Phenylalanine with UVA for the treatment of vitiligo needs more testing for possible side effects. J Am Acad Dermatol. 1999;40(6 Pt 1):1015.

Camacho F, Mazuecos J. Treatment of vitiligo with oral and topical phenylalanine: 6 years of experience. Arch Dermatol. 1999;135(2):216-217.

Cejudo-Ferragud E, Nacher A, Polache A, Ceros-Fortea T, Merino M, Casabo VG. Evidence of competitive inhibition for the intestinal absorption of baclofen by phenylalanine. Int J Pharmaceutics. 1996;132:63-69.

Cormane RH, Siddiqui AH, Westerhof W, Schutgens RB. Phenylalanine and UVA light for the treatment of vitiligo. Arch Dermatol Res. 1985;277(2):126-130.

Fugh-Berman A, Cott JM. Dietary supplements and natural products as psychotherapeutic agents. Psychomatic Med. 1999;61:712-728.

Kovacs SO. Vitiligo. J Am Acad Dermatol. 1998 May;38(5 Pt 1):647-666.

Meyers S. Use of neurotransmitter precursors for treatment of depression. Alt Med Rev. 2000;5(1):64-71.

Pietz J. Neurological aspects of adult phenylketonuria. Curr Opin Neurol. 1998;11:679-688.

Pietz J, Dunckelmann R, Rupp A, et al. Neurological outcome in adult patients with early-treated phenylketonuria. Eur J Pediatr. 1998;157:824-830.

Rezvani I. Defects in metabolism of amino acids; Phenylalanine. In: Behrman RE, Kliefman RM, and Jenson HB, eds. Nelson Textbook of Pediatrics. 16th ed. Philadelphia, PA: W.B. Saunders Company; 2000: 344-346.

Russell AL, McCarty MF. DL-phenylalanine markedly potentiates opiate analgesia - an example of nutrient pharmaceutical up-regulation of the endogenous analgesia system. Med Hypotheses. 2000;55(4):283-288.

Sabelli HC, Fawcett J, Gusovsky F, et al. Clinical studies on the phenylethylamine hypothesis of affective disorder: urine and blood phenylacetic acid and phenylalanine dietary supplements. J Clin Psychiatry. 1986;47:66-70.

Schallreuter KU, Zschiesche M, Moore J, et al. In vivo evidence for compromised phenylalalanine metabolism in vitiligo. Biochem Biophys Res Commun. 1998;243(2):395-399.

Schulpis CH, Antoniou C, Michas T, Strarigos J. Phenylalanine plus ultraviolet light: preliminary report of a promising treatment for childhood vitiligo. Pediat Dermatol. 1989;6(4):332-335.

Shils ME, Olson JA, Shike M, Ross AC. Modern Nutrition in Health and Disease. 9th ed. Baltimore, Md: Williams & Wilkins; 1999(41):1010.

Siddiqui AH, Stolk LM, Bhaggoe R, et al. L-phenylalanine and UVA irradiation in the treatment of vitiligo. Dermatology. 1994;188(3):215-218.

Start K. Treating phenylketonuria by a phenylalanine-free diet. Prof Care Mother Child. 1998;8:109-110.

Walsh NE, Ramamurthy S, Schoenfeld L, Hoffman J. Analgesic effectiveness of D-phenylalanine in chronic pain patients. Arch Phys Med Rehabil. 1986;67(7):436-439.

Werbach MR. Nutritional Influences on Illness. 2nd ed. Tarzana, Calif: Third Line Press; 1993:159-160, 384, 434, 494-495, 506, 580, 613-614, 636.

Woodward WR, Olanow CW, Beckner RM, et al. The effect of L-dopa infusions with and without phenylalanine challenges in parkinsonian patients: Plasma and ventricular CSF L-dopa levels and clinical responses. Neurol. 1993;43:1704-1708.

Review Date: April 2002
Reviewed By: Participants in the review process include: Jacqueline A. Hart, MD, Department of Internal Medicine, Newton-Wellesley Hospital, Harvard University and Senior Medical Editor Integrative Medicine, Boston, MA; Gary Kracoff, RPh (Pediatric Dosing section February 2001), Johnson Drugs, Natick, Ma; Steven Ottariono, RPh (Pediatric Dosing section February 2001), Veteran's Administrative Hospital, Londonderry, NH; Margie Ullmann-Weil, MS, RD, specializing in combination of complementary and traditional nutritional therapy, Boston, MA. All interaction sections have also been reviewed by a team of experts including Joseph Lamb, MD (July 2000), The Integrative Medicine Works, Alexandria, VA;Enrico Liva, ND, RPh (August 2000), Vital Nutrients, Middletown, CT; Brian T Sanderoff, PD, BS in Pharmacy (March 2000), Clinical Assistant Professor, University of Maryland School of Pharmacy; President, Your Prescription for Health, Owings Mills, MD; Ira Zunin, MD, MPH, MBA (July 2000), President and Chairman, Hawaii State Consortium for Integrative Medicine, Honolulu, HI.

Copyright © 2004 A.D.A.M., Inc

The publisher does not accept any responsibility for the accuracy of the information or the consequences arising from the application, use, or misuse of any of the information contained herein, including any injury and/or damage to any person or property as a matter of product liability, negligence, or otherwise. No warranty, expressed or implied, is made in regard to the contents of this material. No claims or endorsements are made for any drugs or compounds currently marketed or in investigative use. This material is not intended as a guide to self-medication. The reader is advised to discuss the information provided here with a doctor, pharmacist, nurse, or other authorized healthcare practitioner and to check product information (including package inserts) regarding dosage, precautions, warnings, interactions, and contraindications before administering any drug, herb, or supplement discussed herein.

  Uses of this Supplement
Parkinson's Disease
  Supplements with Similar Uses
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  Drugs that Interact
Levodopa-containing Medications
  Supplements with Similar Side Effects
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  Supplements with Similar Warnings
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