Drug & supplement interaction

Black Horehound + Carbidopa, Levodopa

Carbidopa, Levodopa brand names: Sinemet CR, Sinemet, Rytary, Dhivy.

Moderate Interaction
Medically reviewed by · Updated September 2026
The bottom line Moderate

Black Horehound has a moderate interaction with Carbidopa, Levodopa. Theoretically, black horehound might have additive effects when used with dopamine agonists.

Read the full interaction report
SeverityModerate
LikelihoodPossible
EvidenceD
What do Severity, Likelihood & Evidence mean?
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Interaction report

Black Horehound and Carbidopa, Levodopa

Here is the interaction we found between these two, with the evidence behind it.

Moderate

DOPAMINE AGONISTS

Why this applies to Carbidopa, Levodopa

Carbidopa, Levodopa falls under DOPAMINE AGONISTS — the group Black Horehound interacts with.

Theoretically, black horehound might have additive effects when used with dopamine agonists. Some constituents of black horehound bind to dopamine D2 receptors in vitro; however, this has not yet been reported in humans. Some dopamine agonists include bromocriptine (Parlodel), levodopa (Dopar, component of Sinemet), pramipexole (Mirapex), ropinirole (Requip), and others.
Severity Moderate Likelihood Possible Evidence D

Interaction 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.

Dr. Brian Staiger, PharmD, BCPS
Dr. Brian Staiger explains PharmD, Clinical Pharmacist

Here's the gist with Sinemet (carbidopa/levodopa) and black horehound. Levodopa works by turning into dopamine in your brain to ease Parkinson's symptoms like tremor, stiffness, and slow movement. In lab dish studies, certain compounds in black horehound have been shown to attach to dopamine D2 receptors, the same kind of receptors dopamine acts on. So in theory, the two could add together and nudge dopamine activity in the same direction.

The key word is theory. This has only been seen in test tubes, not in real people, so we honestly don't know if it matters at the doses folks actually take. If it did add up, you might notice more dopamine-type effects like nausea, dizziness, or extra movements. I'd just mention it to your pharmacist or doctor before combining them, rather than worry.

Contact Dr. Brian directly if you have additional questions on this interaction
References
  1. Daels-Rakotoarison DA, Seidel V, Gressier B, et al. Neurosedative and antioxidant activities of phenylpropanoids from Ballota nigra. Arzneimittelforschung 2000;50:16-23. PubMed

What These Ratings Mean

Every interaction above is graded three ways — here’s the key to each label.

Severity how clinically significant the interaction is
Major

Clinically significant. Avoid the combination, or use only under close professional supervision.

Moderate

Usually worth avoiding or managing — may need monitoring, a dose change, or timing apart.

Minor

Generally not serious. Be aware of it and mention it to your pharmacist.

Likelihood how strong the evidence is that the interaction actually happens in people
  • 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.
Evidence the quality of the studies behind the rating (graded A–D)
  • 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.

Keep it in perspective

Black Horehound Also Interacts With Other Medications

Black Horehound has 22 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.

Moderate 22 No known interaction 2,808 (incl. Carbidopa, Levodopa)
Taking anything else?

Check Another of Your Medications Against Black Horehound

Covers the 2,830 medications we’ve checked against Black Horehound. Pick one to open its full report.

Everything else you take

See Every Supplement That Interacts With Carbidopa, Levodopa

This page covers Black Horehound 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

Open the Carbidopa, Levodopa interaction page

How much people look this up

Where Black Horehound 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.

Supplement Black Horehound
Top half

#467 of 1,452 supplements looked up here in the last 7 days

29 days ago today
Medication Carbidopa, Levodopa
Top 5%

#86 of 2,830 medications looked up here in the last 7 days

29 days ago today

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.

Supplement

About Black Horehound

What is Black Horehound?

Black Horehound
  • Nausea and vomiting
  • Anxiety and nervous tension
  • Sleep problems
  • Mild digestive upset
  • Coughs and respiratory complaints

Black horehound is a traditional European herb mainly used for nausea, mild anxiety, and digestive upset, but high-quality human studies are lacking, so its effectiveness is largely unproven. It has a strong, unpleasant smell and taste, and its safety has not been well studied, so it should be avoided in pregnancy and breastfeeding and used cautiously by everyone else.

Read the full Black Horehound monograph & all interactions
Medication

About Levodopa and Carbidopa

What Is Levodopa and Carbidopa Used For?

Brand names: Sinemet CR, Sinemet, Rytary, Dhivy

Levodopa and carbidopa is used to treat Parkinson's disease, post-encephalitic parkinsonism (parkinsonism that develops after a brain infection), and symptomatic parkinsonism that can follow carbon monoxide or manganese poisoning. These indications are shared across the oral tablet, orally disintegrating tablet, extended-release tablet, and extended-release capsule formulations (including Sinemet, Rytary, Crexont, and generic products).

DUOPA — a carbidopa/levodopa suspension given through a tube directly into the small intestine — is specifically indicated for the treatment of motor fluctuations in patients with advanced Parkinson's disease. This is a product-specific indication and does not apply to the oral forms.

See full Levodopa and Carbidopa drug information
Keep checking

Other Drugs & Black Horehound

Browse every other medication we’ve checked against Black Horehound — both the ones with a known interaction and the ones without. Select any drug for the full detail.

0Major
22Moderate
0Minor
2,807No known interaction
22 Moderate 2,807 No interaction
Moderate

22 interactions

No interaction 2,807 drugs checked — show the list

We also checked Black Horehound against these medications and found no known interaction in our sources. Open any page to confirm.

Showing the 60 most looked-up of 2,807 medications checked with no known interaction — see the full Black Horehound list.

See the full Black Horehound interactions page
See every supplement checked against Carbidopa, Levodopa, ranked by severity
Sources

Sources & How We Checked

Interaction details are evidence-graded and sourced from the Natural Medicines database. The medication overview comes from MedlinePlus.

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

Keep exploring
Black Horehound monograph › Uses, safety, and every drug interaction in one place Carbidopa, Levodopa supplement interactions › Every supplement checked against Carbidopa, Levodopa, ranked by severity Levodopa and Carbidopa drug information › The full plain-English guide to this medication Supplement interaction checker › Check any supplement or product against your medications Browse medications A–Z › Every drug we've checked against herbs & supplements Ask a pharmacist › Free, personal answer from our licensed team
Pharmacist Counseling Corner

Taking Black Horehound and Carbidopa, Levodopa Together: Common Questions

Can you take Black Horehound with Carbidopa, Levodopa?
There is a moderate interaction, so it may be acceptable with monitoring or timing changes — but check with your pharmacist first. Read the full details on this page and confirm with your pharmacist before combining them.
How serious is the Black Horehound and Carbidopa, Levodopa interaction?
We rate it Moderate. This combination may require monitoring, a dose adjustment, or separating the times you take each one. Consult your pharmacist before using them together.

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

Still Have a Question About Carbidopa, Levodopa and Black Horehound?

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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.