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Dietary supplement

L-Phenylalanine 500 mg Ingredients & Drug Interactions

by NOW

L-Phenylalanine 500 mg label
See full label image
Capsule Category: Amino Acid/protein
Most serious interaction: Major
Clinical methodology reviewed by · Updated Sep 16, 2026 · Label data from the NIH DSLD
What is this page for?

First and foremost: checking L-Phenylalanine 500 mg against your medications. The heart of this page is the interaction checker and the full interaction report — how this product’s ingredients may interact with prescription and over-the-counter medicines you may be taking.

Around that, we add a pharmacist’s high-level view of the product as a whole — what’s inside, the evidence for its stated use, how transparent the label is, and what safety data exists — so you can see the full picture in one place. It’s educational information from our licensed clinical databases and the clinical staff at HelloPharmacist — not medical advice — and we don’t sell or endorse products. Our editorial policy

The interaction bottom line Most serious interaction: Major

L-Phenylalanine 500 mg is a dietary supplement by NOW with 1 active ingredient. Its ingredients are commonly taken for depression and mood, skin condition vitiligo, pain conditions.Based on those ingredients, 16 medications have a known interaction with it, the most serious rated major. The ingredients most likely to interact are L-Phenylalanine. Use the checker below to test your specific medication, or read the full HelloPharmacist Interaction Report.

Jump to a section

Check Your Meds Against L-Phenylalanine 500 mg by NOW

Ask about any prescription or over-the-counter medication and we check it for interactions with L-Phenylalanine 500 mg by NOW — and tell you which ingredient is responsible.

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HelloPharmacist Scorecard of L-Phenylalanine 500 mg by NOW

Our pharmacy team’s full take, with four database checks built into the cards below — a summary of what is known, not a grade of the product itself.

From our pharmacy team — supplement deep dive

Generated from HelloPharmacist’s pharmacist-curated databases and editorial rules · Last generated Aug 13, 2026

What’s inside

Full disclosure
Ingredient Transparency · database check
Full

Every active ingredient lists its own amount on the label.

Why this rating?
  • The label discloses an exact amount for 1 of its 1 active ingredient.
  • No proprietary blends here — you can verify the dose of every single component.

This supplement contains one active ingredient: L-phenylalanine, an amino acid your body normally gets from protein-containing foods. The capsule also contains cellulose, an inactive filler.

Each capsule delivers 500 mg of L-phenylalanine.

Does it work?

Not established
Evidence for Intended Use · database check
By FDA rules, dietary supplements can’t claim to treat, cure, or prevent disease — so labels speak in careful marketing language. We discern each product’s intended use from its name, label claims, and label statements, then grade the clinical evidence for that use. How these ratings are computed
Not established

The graded evidence we hold for these ingredients covers different conditions than the ones this product is marketed for, so there's no established rating for its stated use — see Phenylalanine's own graded uses on its monograph.

Why this rating?
  • The label markets this product for: amino acid for neurotransmitter synthesis support.
  • We looked for evidence on: Depression, Attention deficit-hyperactivity disorder (ADHD), Parkinson disease, mood, dopamine production, cognitive function.
  • The closest evidence on file: Phenylalanine is rated "Possibly Ineffective" for Attention deficit-hyperactivity disorder (ADHD) (Natural Medicines).
  • Also on file: Phenylalanine is rated "Insufficient Reliable Evidence To Rate" for Depression, Parkinson disease.

The evidence for L-phenylalanine is mixed and limited. It's rated possibly effective for vitiligo (a skin condition involving loss of pigment).

For ADHD, the data suggests it's possibly ineffective. For alcohol use disorder and multiple sclerosis, there isn't enough reliable evidence in our data to say whether it helps or doesn't.

If you're considering this supplement for a specific condition, talk it over with your doctor or pharmacist about what the research actually shows for your situation.

The evidence, ingredient by ingredient Phenylalanine

How safe is it?

Well-documented data
Safety Information · database check
Well characterized

Adverse-effect, pregnancy, and general safety data are on file for most of these ingredients.

Why this rating?
  • We hold adverse-effect (side-effect) data for 1 of the 1 matched ingredient.
  • Pregnancy & breastfeeding safety ratings cover 1 of 1.
  • General safety write-ups exist for 1 of 1.
  • Remember: this measures how much safety information exists. Thin data is not the same as being safe.

L-Phenylalanine is generally well tolerated at typical doses when you get it from food, though supplement doses are a different story. The most common side effects from oral supplements are anxiety, constipation, headache, heartburn, insomnia, nausea, and sedation.

Do not use this supplement if you have phenylketonuria (PKU), a rare genetic condition in which your body cannot process phenylalanine safely. If you are pregnant or breastfeeding, safety data for supplement doses isn't established, so talk with your doctor or pharmacist before taking this.

Side effects, ingredient by ingredient Phenylalanine

Meds to double-check

Major interaction found
Known Interaction Concern · database check
Major identified

At least one ingredient has a documented Major-severity interaction. Check your medications for a personalized result.

Why this rating?
  • 1 of the 1 matched ingredient can interact with medications — Phenylalanine.
  • The most serious interaction on file is rated Major.
  • Some involve high-stakes drug classes: Parkinson's medications.
  • For scale: 16 individual medications appear in the full list. A big number alone doesn't make a product dangerous — what matters is whether YOUR medication is on it, so run yours through the interaction checker on this page.

Before taking L-Phenylalanine, double-check with your doctor or pharmacist if you take levodopa (it's a Major interaction affecting how the drug works in your brain), baclofen (Moderate — can lower absorption), or a monoamine oxidase inhibitor MAOI antidepressant (Moderate — potential blood pressure risk). No interactions are documented for other ingredient combinations we could not verify, but your own medications may still need a check.

Check your own medication Run your meds through the checker above

The bottom line

Scorecard at a glanceFully disclosed formula with no established evidence rating for its marketed use. Major medication interactions have been identified, and safety information is well characterized.

L-Phenylalanine might be worth considering if you're exploring options for vitiligo and your doctor approves, but it's not a proven tool for ADHD and evidence for other uses is scarce. If you take levodopa for Parkinson disease, baclofen for muscle tension, or an MAOI antidepressant, this supplement could interfere with your medication — check with your own doctor or pharmacist first.

Anyone with PKU must avoid this product entirely.

Educational only — not medical advice; always confirm with your pharmacist. Our editorial policy · How we use AI

Assessment coverage: 1 of 1 active ingredient matched to our full ingredient reviews (monographs). Based on the product label dated Jun 23, 2017.

This Scorecard evaluates available label information, ingredient evidence, and known medication-safety considerations. It does not independently verify product identity, purity, potency, contamination, or manufacturing quality. How these ratings are computed

At a glance

General information

Key facts about L-Phenylalanine 500 mg, straight from the product label.

Brand NOW
Barcode (UPC) 733739001306
Net contents 60 Veg Capsule(s)
Market status On market
Date entered into DSLD Jun 23, 2017
DSLD ID 74063
Product type Amino Acid/protein
Supplement form Capsule
Dietary claims / uses All Other, Structure/Function
Intended target group(s) Vegan, Vegetarian, Adult (18 - 50 Years), Gluten Free, Dairy Free
From the label
Everything in this section is reproduced from the manufacturer’s own product label — it’s the label speaking, not HelloPharmacist. We show it so you can see exactly what the maker states; we don’t verify or endorse those statements.

Supplement Facts

The label details for L-Phenylalanine 500 mg by NOW, sourced from the NIH Dietary Supplement Label Database.

Supplement Facts

Daily Value (DV) Target Group(s):
Adults and children 4 or more years of age
Minimum serving Sizes:
1 Veg Capsule(s)
Maximum serving Sizes:
1 Veg Capsule(s)
UPC/BARCODE
733739001306
IngredientAmount% DV
L-Phenylalanine500 mg--

Other ingredients: Cellulose

Tap any ingredient to jump to its full detail below.

Label statements
These statements are the manufacturer’s wording, reproduced from the product label — the label is saying it, not HelloPharmacist. We don’t verify or endorse them.
Suggested/Recommended/Usage/Directions

Suggested Usage:Take 1 capsule 1 to 3 times daily as needed, preferably between meals.

General Statements

L-Phenylalanine is an essential amino acid that is necessary for the synthesis of proteins.* Because L-Phenylalanine can be readily converted into the amino acid tyrosine, which forms part of the structure of the neurotransmitters dopamine and norepinephrine, it is important for the maintenance of a healthy nervous system.*

Natural color variation may occur in this product.

Nervous system support* Free-form

Amino Acids Family owned since 1968.

Settling of powder may occur in the capsule, which is filled by weight without added fillers.

Please recycle.

FDA Disclaimer Statement

*These statements have not been evaluated by the Food and Drug Administration. This product is not intended to diagnose, treat, cure or prevent any disease.

Precautions

Caution: For adults only.

Consult physician if pregnant/nursing, taking medication, or have a medical condition [e.g. high blood pressure or phenylketonuria (PKU)].

Keep out of reach of children.

Not manufactured with wheat, gluten, soy, milk, egg, fish, shellfish or tree nut ingredients. Produced in a GMP facility that processes other ingredients containing these allergens.

General

CODE 0130C V6

Seals/Symbols

Non-GMO

GMP Quality Assured

Formulation

Non-GMO

Vegetarian/Vegan

Not manufactured with wheat, gluten, soy, milk, egg, fish, shellfish or tree nut ingredients.

FDA Statement of Identity

A Dietary Supplement

Storage

Store in a cool, dry place after opening.

Is this label outdated? Report a formula or label change and our pharmacy team will review it.

See for yourself

L-Phenylalanine 500 mg by NOW label

The label scan from the NIH Dietary Supplement Label Database. Tap to enlarge.

L-Phenylalanine 500 mg label

Label images are published by the NIH Dietary Supplement Label Database for the version of this product on file. Always read your actual product label.

View the full label (PDF)
What’s inside

The Ingredients in L-Phenylalanine 500 mg by NOW

This is the 1 active ingredient this product is made of. Select it to open its full monograph.

Serving size1 Veg Capsule(s) Dosage formCapsule Amounts shown are per serving.

L-Phenylalanine

Interacts with
16 drugs
500 mg per serving

Phenylalanine is an essential amino acid the body uses to make brain chemicals like dopamine and norepinephrine. Some people take it for mood, vitilig...

L-Phenylalanine monograph & interactions

Other (inactive) ingredients: Cellulose. These complete the product’s ingredient list but are not active constituents.

Interaction report

L-Phenylalanine 500 mg by NOW Drug Interactions

Want to check YOUR meds against L-Phenylalanine 500 mg?

Ask about interactions with your drugs in plain English — “Can I take it with lisinopril?” — and we find you the answer in seconds, ingredient by ingredient.

 Go to the checker
16Drugs
5 Major 11 Moderate

Ingredients driving the most interactions

L-Phenylalanine 16

Individual Drug Interactions

The ingredients in L-Phenylalanine 500 mg interact with 16 drugs. Click any drug to see the details.

1 of the 1 ingredient in L-Phenylalanine 500 mg interact with drugs. Each result below shows which ingredient is responsible. L-Phenylalanine

Each ingredient & the kinds of drugs it affects

For each ingredient in L-Phenylalanine 500 mg with known interactions, here are the types of medications it can affect. Open any type for the detail — or search your exact drug in the checker above.

L-Phenylalanine3 drug types · 16 drugs

Levodopa

Phenylalanine, especially in high doses, can reduce the effectiveness of levodopa.
Phenylalanine competes with levodopa for carrier-mediated transport into the brain. The resulting reduction in levels of levodopa in the brain can exacerbate tremor, rigidity, and the "on-off" phenomenon in patients with Parkinson disease.

Likelihood Probable Evidence B
Baclofen

Concomitant intake of phenylalanine may reduce the intestinal absorption of baclofen.
Phenylalanine and baclofen share the same intestinal carrier for absorption; phenylalanine competitively inhibits the absorption of baclofen, reducing its plasma levels.

Likelihood Possible Evidence D
Monoamine Oxidase Inhibitors (Maois)

Theoretically, concomitant use of L-phenylalanine and non-selective MAOIs might increase the risk of hypertensive crisis.
L-phenylalanine is metabolized to tyrosine. Some evidence suggests that L-phenylalanine, given with the non-selective MAOI pargyline, might prevent the elimination of tyramine, increasing the risk of hypertensive crisis. However, this was not reported in a small number of patients when using L-phenylalanine with the partially selective MAO-B inhibitor, selegiline.

Likelihood Possible Evidence D
L-Phenylalanine full monograph & interactions
The maker

Brand information

Manufacturer and brand details for L-Phenylalanine 500 mg, from the product label.

NOW

See all NOW products
Name
NOW Foods
Street Address
395 S. Glen Ellyn Rd.
City
Bloomingdale
State
IL
ZipCode
60108
Web Address
nowfoods.com
Pharmacist Counseling Corner

L-Phenylalanine 500 mg by NOW: Common Questions

Does L-Phenylalanine 500 mg by NOW interact with any medications?
Yes. Based on its ingredients, L-Phenylalanine 500 mg has a known interaction with 16 medications, including 5 rated major. Use the checker to see how it interacts with a specific drug.
How can one product interact with so many drugs?
L-Phenylalanine 500 mg contains a single active ingredient, and that one ingredient can interact with many different medications on its own. We check it against each medication and show the mechanism responsible.
Where does this information come from?
The product label data comes from the NIH Dietary Supplement Label Database (DSLD); the interaction data is built on the Natural Medicines database and reviewed by HelloPharmacist pharmacists.
Can I take this if I'm pregnant or breastfeeding?
Safety data for L-Phenylalanine supplements during pregnancy and breastfeeding isn't established, so the facts advise against supplement doses in both situations. Talk with your doctor or pharmacist before taking it if you're pregnant or nursing — they can give you personalized advice based on your health.
What are the most common side effects?
When taken orally at typical doses, the most common side effects are anxiety, constipation, headache, heartburn, insomnia, nausea, and sedation. Most people tolerate it well, but if you're sensitive to any of these, you might want to start low or skip it.
Is this effective for ADHD?
The evidence suggests L-Phenylalanine is possibly ineffective for ADHD, so it's not a reliable choice for that condition. If you're looking for ADHD support, talk with your doctor about options with stronger evidence behind them.
Why can't I take this if I have PKU?
PKU (phenylketonuria) is a genetic condition where your body can't process phenylalanine safely, causing it to build up to harmful levels. This supplement is pure phenylalanine, so it's unsafe if you have PKU — your doctor or pharmacist can advise you on what you should avoid.
Does this supplement have any fillers or other inactive ingredients?
Yes, the capsule contains cellulose as an inactive ingredient — it's a common binder and filler. The only active ingredient is the L-phenylalanine itself.

Written and reviewed by the HelloPharmacist editorial staff. Our editorial policy

Not sure if L-Phenylalanine 500 mg is safe with your meds?

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Label information is sourced from the NIH Dietary Supplement Label Database and reflects the product version on file; always read your actual product label. This page is for education only and is not a substitute for professional medical advice. Confirm with your pharmacist or doctor before combining supplements and medications.

L-Phenylalanine 500 mg label
Go deeper

The Full Monographs Behind L-Phenylalanine 500 mg’s Ingredients

Every ingredient we hold a full HelloPharmacist monograph for — uses, evidence, safety, and the complete interaction list.

Herb & supplement monograph

Phenylalanine

Interacts with 16 drugs

Phenylalanine is an essential amino acid the body uses to make brain chemicals like dopamine and norepinephrine. Some people take it for mood, vitiligo, or pain, but the evidence is mostly l...

Read the full Phenylalanine monograph →
Sources

Sources & How We Checked

L-Phenylalanine 500 mg's label data comes from the NIH Dietary Supplement Label Database; the ingredient interaction data is from the Natural Medicines database, reviewed by our pharmacists.

Content is written and reviewed by licensed HelloPharmacist pharmacists. See our data sources and editorial standards for how this information is built and checked.

The 23 references behind this product’s interaction data

Every citation that drives the interaction findings for this product’s ingredients, from the evidence-graded Natural Medicines (TRC Healthcare) database. Open an ingredient to browse its citations — links open the study on PubMed or the publisher’s site.

Phenylalanine 23 references
  1. Rouse B, Azen C, Koch R, et al. Maternal phenylketonuria collaborative Study (MPKUCS) offspring: facial anomalies, malformations, and early neurological sequelae. Am J Med Genet 1997;69:89-95. DOI
  2. Sturtevant FM. Use of aspartame in pregnancy. Int J Fertil 1985;30:85-7.
  3. Silkaitis RP, Mosnaim AD. Pathways linking L-phenylalanine and 2-phenylethylamine with p-tyramine in rabbit brain. Brain Res 1976;114:105-15.
  4. Lehmann WD, Theobald N, Fischer R, Heinrich HC. Stereospecificity of phenylalanine plasma kinetics and hydroxylation in man following oral application of a stable isotope-labelled pseudo-racemic mixture of L- and D-phenylalanine. Clin Chim Acta 1983;128 PubMed
  5. Mosnik DM, Spring B, Rogers K, Baruah S. Tardive dyskinesia exacerbated after ingestion of phenylalanine by schizophrenic patients. Neuropsychopharmacology 1997;16:136-46. PubMed
  6. Siddiqui AH, Stolk LM, Bhaggoe R, et al. L-phenylalanine and UVA irradiation in the treatment of vitiligo. Dermatology 1994;88:215-8. PubMed
  7. Birkmayer W, Riederer P, Linauer W, Knoll J. L-deprenyl plus L-phenylalanine in the treatment of depression. J Neural Transm 1984;59:81-7. PubMed
  8. Nutt JG, Woodward WR, Hammerstad JP, et al. The "on-off" phenomenon in Parkinson's disease. Relation to levodopa absorption and transport. N Engl J Med 1984;310:483-8. PubMed
  9. Baruzzi A, Contin M, Riva R, et al. Influence of meal ingestion time on pharmacokinetics of orally administered levodopa in parkinsonian patients. Clin Neuropharmacol 1987;10:527-37. PubMed
  10. Juncos JL, Fabbrini G, Mouradian MM, et al. Dietary influences on the antiparkinsonian response to levodopa. Arch Neurol 1987;44:1003-5. PubMed
  11. Eriksson T, Granerus AK, Linde A, et al. "On-off" phenomenon in Parkinson's disease: relationship between dopa and other large neutral amino acids in plasma. Neurology 1988;38:1245-8. PubMed
  12. Baker GB, Bornstein RA, Rouget AC, et al. Phenylethylaminergic mechanisms in attention-deficit disorder. Biol Psychiatry 1991;29:15-22.. PubMed
  13. Wood DR, Reimherr FW, Wender PH. Treatment of attention deficit disorder with DL-phenylalanine. Psychiatry Res 1985;16:21-6.. PubMed
  14. Food and Nutrition Board, Institute of Medicine. Dietary Reference Intakes for Energy, Carbohydrate, Fiber, Fat, Fatty Acids, Cholesterol, Protein, and Amino Acids (Macronutrients). Washington, DC: National Academy Press, 2002. Available at: http://www.n
  15. Cederbaum S. Phenylketonuria: an update. Curr Opin Pediatr 2002;14:702-6. PubMed
  16. Cejudo-Ferragud, E., Nacher, A., Polache, A., Cercos-Fortea, T., Merino, M., and Casabo, V. G. Evidence of competitive inhibition for the intestinal absorption of baclofen by phenylalanine. Int J of Pharm (Amsterdam) 1996;132:63-69. DOI
  17. Fischer, E., Heller, B., Nachon, M., and Spatz, H. Therapy of depression by phenylalanine. Preliminary note. Arzneimittelforschung. 1975;25(1):132.
  18. Beckmann, H., Strauss, M. A., and Ludolph, E. Dl-phenylalanine in depressed patients: an open study. J.Neural Transm. 1977;41(2-3):123-134. PubMed
  19. Sabelli, H. C., Fawcett, J., Gusovsky, F., Javaid, J. I., Wynn, P., Edwards, J., Jeffriess, H., and Kravitz, H. Clinical studies on the phenylethylamine hypothesis of affective disorder: urine and blood phenylacetic acid and phenylalanine dietary supplem
  20. Cotzias, G. C., Van Woert, M. H., and Schiffer, L. M. Aromatic amino acids and modification of parkinsonism. N Engl.J Med 2-16-1967;276(7):374-379. PubMed
  21. Kravitz, H. M., Sabelli, H. C., and Fawcett, J. Dietary supplements of phenylalanine and other amino acid precursors of brain neuroamines in the treatment of depressive disorders. J Am Osteopath.Assoc 1984;84(1 Suppl):119-123. DOI
  22. Mann, J., Peselow, E. D., Snyderman, S., and Gershon, S. D-phenylalanine in endogenous depression. Am.J.Psychiatry 1980;137(12):1611-1612. PubMed
  23. Katoulis AC, Alevizou A, Bozi E, et al. A randomized, double-blind, vehicle-controlled study of a preparation containing undecylenoyl phenylalanine 2% in the treatment of solar lentigines. Clin Exp Dermatol 2010;35(5):473-6. PubMed

See these in context on the Phenylalanine monograph →

Parts of this content are provided by the Therapeutic Research Center, LLC.

DISCLAIMER: Currently this does not check for drug-drug interactions. This is not an all-inclusive comprehensive list of potential interactions and is for informational purposes only. Not all interactions are known or well-reported in the scientific literature, and new interactions are continually being reported. Input is needed from a qualified healthcare provider including a pharmacist before starting any therapy. Application of clinical judgment is necessary.

© 2021 Therapeutic Research Center, LLC

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