Phenylalanine + Carbidopa, Levodopa
Carbidopa, Levodopa brand names: Sinemet CR, Sinemet, Rytary, Dhivy.
Phenylalanine has a major interaction with Carbidopa, Levodopa. Phenylalanine, especially in high doses, can reduce the effectiveness of levodopa.
Read the full interaction reportPhenylalanine and Carbidopa, Levodopa
Here is the interaction we found between these two, with the evidence behind it.
LEVODOPA
Why this applies to Carbidopa, Levodopa
Carbidopa, Levodopa falls under LEVODOPA — the group Phenylalanine interacts with.
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 to know about this combo. Both phenylalanine and levodopa are amino acids, and they actually compete for the same "shuttle" that carries them across into the brain. If you're taking phenylalanine in concentrated supplement doses, it can crowd out levodopa, meaning less of your medicine reaches the brain where it's needed to make dopamine.
For someone with Parkinson's, that can show up as your medication working less well: more tremor, stiffness, slowness, or rougher "on-off" swings where you feel like your dose wears off too soon. This is rated a major interaction with decent evidence, so it's worth taking seriously. Everyday food protein has some phenylalanine too, which is why meal timing matters with levodopa. Before adding a phenylalanine supplement, please check with your doctor or pharmacist first.
- 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 Carbidopa, Levodopa 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 Carbidopa, Levodopa
This page covers Phenylalanine only. If you take other supplements — a multivitamin, a sleep aid, something for joints — the Carbidopa, Levodopa page lists 43 supplements it has a documented interaction with, worst first, each one explained.
The most serious there right now: Activated Charcoal (major), Branched-chain Amino Acids (bcaa) (major), Cowhage (major), Indian Snakeroot (major) and Magnesium (major).
By severity: 8 major · 24 moderate · 11 minor.
From our pharmacist’s overview on the Carbidopa, Levodopa page
“Carbidopa/levodopa (Sinemet, Rytary) anchors Parkinson's treatment, and because symptom control hinges on how much levodopa actually reaches the brain, the 43 supplements and herbs on this list deserve real respect before you add any of them. Eight carry a major rating, and almost all of the serious entries weaken the drug rather than…”
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 Carbidopa, Levodopa 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 Carbidopa, Levodopa 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
#266 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.
About Levodopa and Carbidopa
What Is Levodopa and Carbidopa Used For?
Brand names: Sinemet CR, Sinemet, Rytary, Dhivy
Levodopa and carbidopa treats Parkinson’s disease, post-encephalitic parkinsonism (parkinsonism after brain inflammation), and parkinsonism that may follow carbon monoxide or manganese intoxication. This applies to the oral tablets, orally disintegrating tablets, and extended-release tablets and capsules.
Duopa (enteral suspension) is used specifically for motor fluctuations in people with advanced Parkinson’s disease. Crexont is labeled for adults.
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
Carbidopa, Levodopa Is Linked to Lower Levels of 2 Nutrients
Natural Medicines records Carbidopa, Levodopa 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 Carbidopa, Levodopa 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 Carbidopa, Levodopa Together: Common Questions
Can you take Phenylalanine with Carbidopa, Levodopa?
How serious is the Phenylalanine and Carbidopa, Levodopa 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.