Phenylalanine + Levodopa, Carbidopa
Levodopa, Carbidopa brand name: Duodopa.
Phenylalanine has a major interaction with Levodopa, Carbidopa. Phenylalanine, especially in high doses, can reduce the effectiveness of levodopa.
Read the full interaction reportPhenylalanine and Levodopa, Carbidopa
Here is the interaction we found between these two, with the evidence behind it.
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.
Interaction has not been documented in well-controlled studies, however, the interaction has been demonstrated in some small human studies or in controlled animal studies in conjunction with multiple case reports..
Here's the key thing about your Parkinson's medicine: levodopa has to hitch a ride into your brain on a special transporter that also carries amino acids. Phenylalanine, especially in concentrated supplement doses, competes for that same ride. When phenylalanine wins the seat, less levodopa actually reaches your brain, even though you took your normal dose.
In real life, that can mean your Parkinson's symptoms creep back: more tremor, stiffness, slowness, or a rockier on-off pattern through the day. The carbidopa part isn't the issue here, it's the levodopa getting crowded out. Normal amounts of protein in food are usually manageable, but a phenylalanine supplement is a different story. Please don't stop anything on your own, just talk with your pharmacist or doctor before adding it.
- 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
- 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
What These Ratings Mean
Every interaction above is graded three ways — here’s the key to each label.
Clinically significant. Avoid the combination, or use only under close professional supervision.
Usually worth avoiding or managing — may need monitoring, a dose change, or timing apart.
Generally not serious. Be aware of it and mention it to your pharmacist.
- LikelyWell-controlled human studies have demonstrated the likely existence of this interaction.
- ProbableInteraction has not been documented in well-controlled studies, however, the interaction has been demonstrated in some small human studies or in controlled animal studies in conjunction with multiple case reports.
- PossibleInteraction has been documented in animal or in lab research, or the interaction has been documented in humans but is limited to case reports or conflicting clinical research exists.
- UnlikelyInteraction has been demonstrated in animal or in lab research but has been shown not to occur in humans.
- AHigh-quality meta-analysis (quantitative systematic review).
- BCase-control study.
- CConsensus.
- DAnecdotal evidence.
These ratings are for information only. Never start, stop, or otherwise change your therapy before speaking with your provider.
Phenylalanine Also Interacts With Other Medications
Phenylalanine has 4 major, 11 moderate known drug interactions in our database — the Levodopa, Carbidopa one above is one of them. If you take anything else, check it against the full list below.
Check Another of Your Medications Against Phenylalanine
We haven’t checked that one against Phenylalanine yet — ask our pharmacists directly.
Covers the 2,830 medications we’ve checked against Phenylalanine. Pick one to open its full report.
See Every Supplement That Interacts With Levodopa, Carbidopa
This page covers Phenylalanine only. If you take other supplements — a multivitamin, a sleep aid, something for joints — the Levodopa, Carbidopa page lists 26 supplements it has a documented interaction with, worst first, each one explained.
The most serious there right now: Branched-chain Amino Acids (bcaa) (major), Cowhage (major), Indian Snakeroot (major), Magnesium (major) and Rauvolfia Vomitoria (major).
By severity: 7 major · 17 moderate · 2 minor.
From our pharmacist’s overview on the Levodopa, Carbidopa page
“Cowhage is the first thing to know here. It naturally contains levodopa, sometimes 75 to 400 mg per dose, so stacking it on Levodopa, Carbidopa (Duodopa) risks levodopa excess.”
Reviewed by 2 · Jul 18, 2026
Taking Phenylalanine as Part of a Dietary Supplement Product? Check It for Interactions
We hold 1,990 real supplement products containing Phenylalanine, each one broken down to its full ingredient list — and combination products often carry several ingredients besides Phenylalanine. Your product may carry other ingredients that interact with Levodopa, Carbidopa too, on top of Phenylalanine — and those would not show on this page. Find your product and we’ll check every ingredient on its label:
Where Phenylalanine and Levodopa, Carbidopa Sit Against Everything Else
Out of every supplement and medication people look up on HelloPharmacist, here is where these two land this week — and which way each has been moving.
#126 of 1,452 supplements looked up here in the last 7 days
#387 of 2,830 medications looked up here in the last 7 days
Position among everything looked up on this site, not a measure of how commonly either one is used or how important it is. A supplement can climb this list because of a news story. It counts every request to the page, automated ones included, and nothing here identifies anyone.
About Phenylalanine
What is Phenylalanine?
- Depression and mood
- Skin condition vitiligo
- Pain conditions
- Attention and focus
- Appetite control
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 limited or mixed. People with the genetic condition PKU must strictly avoid it.
Other Drugs & Phenylalanine
Browse every other medication we’ve checked against Phenylalanine — both the ones with a known interaction and the ones without. Select any drug for the full detail.
No drugs match “”.
4 interactions
11 interactions
No interaction 2,814 drugs checked — show the list
We also checked Phenylalanine against these medications and found no known interaction in our sources. Open any page to confirm.
- Apixaban ›
- Levothyroxine ›
- Amlodipine ›
- Methylphenidate ›
- Warfarin ›
- Abemaciclib ›
- Ivermectin ›
- Testosterone ›
- Sertraline ›
- Losartan Potassium ›
- Acetaminophen ›
- Aspirin ›
- Rosuvastatin ›
- Metoprolol ›
- Escitalopram ›
- Clopidogrel ›
- Rivaroxaban ›
- Atorvastatin ›
- Isotretinoin ›
- Warfarin Sodium ›
- Tirzepatide ›
- Amlodipine Besylate ›
- Ibuprofen ›
- Methamphetamine ›
- Docusate ›
- Tadalafil ›
- Hydroxyurea ›
- Metformin ›
- Valacyclovir ›
- Sumatriptan ›
- Prednisone ›
- Semaglutide ›
- Amitriptyline ›
- Lisdexamfetamine ›
- Levothyroxine, Liothyronine ›
- Propranolol ›
- Methotrexate ›
- Amoxicillin ›
- Lisinopril ›
- Bupropion ›
- Fluoxetine ›
- Azithromycin ›
- Bendamustine ›
- Pregabalin ›
- Doxycycline ›
- Gabapentin ›
- Dextroamphetamine ›
- Lamotrigine ›
- Acyclovir ›
- Acetaminophen, Aspirin, Caffeine ›
- Naproxen ›
- Quetiapine ›
- Clonazepam ›
- Buspirone ›
- Omeprazole ›
- Duloxetine ›
- Letrozole ›
- Citalopram ›
- Methylene Blue ›
- Acetaminophen, Codeine ›
Showing the 60 most looked-up of 2,814 medications checked with no known interaction — see the full Phenylalanine list.
Sources & How We Checked
Interaction details are evidence-graded and sourced from the Natural Medicines database. The medication overview comes from MedlinePlus.
- Natural Medicines (Therapeutic Research Center) — Evidence-graded clinical reference for vitamins, herbs and supplements — the basis for our interaction data.
- MedlinePlus, U.S. National Library of Medicine — NIH consumer drug information, used for the medication overviews.
- NIH Dietary Supplement Label Database (DSLD) — Official supplement product labels.
Content is written and reviewed by licensed HelloPharmacist pharmacists. See our data sources and editorial standards for how this information is built and checked.
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
Levodopa, Carbidopa Is Linked to Lower Levels of 2 Nutrients
Natural Medicines records Levodopa, Carbidopa as a medicine that can lower these. This is not an interaction with Phenylalanine — it is a separate effect of the medicine, and it is a reason some people are advised to supplement rather than to avoid one:
Read the full Levodopa, Carbidopa nutrient depletion report ›
Depletion records come from the licensed Natural Medicines database. Do not start a supplement on the strength of this page — ask your pharmacist or doctor whether it applies to you.
Taking Phenylalanine and Levodopa, Carbidopa Together: Common Questions
Can you take Phenylalanine with Levodopa, Carbidopa?
How serious is the Phenylalanine and Levodopa, Carbidopa interaction?
Written and reviewed by the HelloPharmacist editorial staff. Our editorial policy
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This information is for education only and is not a substitute for professional medical advice. Always check with your pharmacist or doctor before starting, stopping, or combining supplements and medications.