Drug & supplement interaction

Syrian Rue + Dextromethorphan HBr, Guaifenesin

Dextromethorphan HBr, Guaifenesin brand names: Vicks Formula 44E Cough & Chest Congestion Relief, Pediatric Vicks Formula 44e Cough & Chest Congesti.

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

Dextromethorphan HBr, Guaifenesin falls into 3 drug categories that Syrian Rue can interact with; the most serious is rated moderate. Theoretically, Syrian rue might increase the risk of serotonergic adverse effects, including serotonin syndrome, when taken with serotonergic drugs.

Read the full interaction report
SeverityModerate
LikelihoodPossible
Interactions3
EvidenceD
What do Severity, Likelihood & Evidence mean?
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Interaction report

Syrian Rue and Dextromethorphan HBr, Guaifenesin

These two interact in 3 distinct ways. Each one is explained separately below.

Why Are There 3 Interactions?

Syrian Rue interacts with whole categories of medication. Dextromethorphan HBr, Guaifenesin falls into 3 of those categories, so it can interact with Syrian Rue in 3 different ways. We’ve listed each one below — your overall risk is driven by the most serious of them (Moderate).

Moderate

SEROTONERGIC DRUGS

1 of 3

Why this applies to Dextromethorphan HBr, Guaifenesin

Dextromethorphan HBr, Guaifenesin falls under SEROTONERGIC DRUGS — the group Syrian Rue interacts with.

Theoretically, Syrian rue might increase the risk of serotonergic adverse effects, including serotonin syndrome, when taken with serotonergic drugs.
Syrian rue contains harmaline and harmine, which have demonstrated strong, reversible inhibition of monoamine oxidase (MAO) A and weak inhibition of MAO B in animal and in vitro research. This may increase the risk for adverse effects with serotonergic drugs.
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

Let's talk about that cough syrup. The dextromethorphan in it nudges up serotonin in your brain, and Syrian Rue acts like an MAO inhibitor, meaning it blocks the enzyme that normally clears serotonin away. Stack the two together and serotonin can climb too high. That raises the risk of what we call serotonin syndrome, with symptoms like agitation, a racing heart, sweating, shivering, muscle twitching, or confusion. The guaifenesin part is just a mucus thinner and isn't the concern here.

To be fair, this warning is mostly theoretical, based on lab and animal studies, not strong human reports. Still, combining DXM with any MAOI is a classic caution. I'd steer clear of Syrian Rue and check with your pharmacist before using both.

Contact Dr. Brian directly if you have additional questions on this interaction
References
  1. Kawanishi, K., Hashimoto, Y., Fujiwara, M., Kataoka, Y., and Ueki, S. Pharmacological characteristics of abnormal behavior induced by harmine with special reference to tremor in mice. J Pharmacobiodyn 1981;4(7):520-527. PubMed
  2. Herraiz, T., Gonzalez, D., Ancin-Azpilicueta, C., Aran, V. J., and Guillen, H. beta-Carboline alkaloids in Peganum harmala and inhibition of human monoamine oxidase (MAO). Food Chem Toxicol 2010;48(3):839-845. PubMed
  3. Fuller, R. W., Wong, C. J., and Hemrick-Luecke, S. K. MD 240928 and harmaline: opposite selectivity in antagonism of the inactivation of types A and B monoamine oxidase by pargyline in mice. Life Sci 1986;38(5):409-412. PubMed
  4. Abdel-Fattah, A. F., Matsumoto, K., Murakami, Y., Adel-Khalek, Gammaz H., Mohamed, M. F., and Watanabe, H. Central serotonin level-dependent changes in body temperature following administration of tryptophan to pargyline- and harmaline-pretreated rats. Ge PubMed
  5. Bergstrom, M., Westerberg, G., and Langstrom, B. 11C-harmine as a tracer for monoamine oxidase A (MAO-A): in vitro and in vivo studies. Nucl Med Biol 1997;24(4):287-293. PubMed
  6. Gerardy, J. Effect of moclobemide on rat brain monoamine oxidase A and B: comparison with harmaline and clorgyline. Prog Neuropsychopharmacol Biol Psychiatry 1994;18(4):793-802.
  7. Sacher, J., Houle, S., Parkes, J., Rusjan, P., Sagrati, S., Wilson, A. A., and Meyer, J. H. Monoamine oxidase A inhibitor occupancy during treatment of major depressive episodes with moclobemide or St. John's wort: an [11C]-harmine PET study. J Psychiatry
Moderate

CYTOCHROME P450 2D6 (CYP2D6) SUBSTRATES

2 of 3

Dextromethorphan HBr, Guaifenesin also falls under CYTOCHROME P450 2D6 (CYP2D6) SUBSTRATES.

Theoretically, Syrian rue might increase the levels and clinical effects of drugs metabolized by CYP2D6.
Syrian rue contains beta-carboline alkaloids, including harmine, harmaline, harmalol, harmol, and harmane. In vitro research suggests that these beta-carboline alkaloids can inhibit CYP2D6 enzymes.
Severity Moderate Likelihood Probable Evidence D

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..

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

Here's the gist with this cough and chest combo. The guaifenesin part (the mucus thinner) isn't really the concern. The dextromethorphan is, and for two reasons. First, your body relies on an enzyme called CYP2D6 to clear dextromethorphan. Syrian Rue may slow that enzyme down, so the drug could build up and hit harder than expected, meaning more drowsiness, dizziness, or that foggy, off-balance feeling.

Second, and bigger: Syrian Rue acts like an MAO inhibitor, and mixing MAOIs with dextromethorphan can trigger serotonin syndrome (agitation, racing heart, sweating, confusion). That combination is genuinely risky. This is based on how the chemistry works rather than solid human studies, but please don't pair these without checking with your pharmacist or doctor first.

Contact Dr. Brian directly if you have additional questions on this interaction
References
  1. Zhao, T., He, Y. Q., Wang, J., Ding, K. M., Wang, C. H., and Wang, Z. T. Inhibition of human cytochrome P450 enzymes 3A4 and 2D6 by β-carboline alkaloids, harmine derivatives. Phytother Res 2011;25(11):1671-1677.
Moderate

CYTOCHROME P450 3A4 (CYP3A4) SUBSTRATES

3 of 3

Dextromethorphan HBr, Guaifenesin also falls under CYTOCHROME P450 3A4 (CYP3A4) SUBSTRATES.

Theoretically, Syrian rue might increase the levels and clinical effects of drugs metabolized by CYP3A4.
Syrian rue contains beta-carboline alkaloids, including harmine, harmol, and harmane. In vitro research suggests that these beta-carboline alkaloids competitively inhibit CYP3A4 enzymes.
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 situation with your cough syrup. The dextromethorphan part is cleared in part by a liver enzyme called CYP3A4. Lab studies suggest the harmala alkaloids in Syrian Rue can slow that enzyme down, so in theory dextromethorphan could build up and hit harder, leaving you more drowsy, dizzy, or even a bit confused. The guaifenesin (the mucus thinner) isn't really the concern here.

There's a bigger worry, though. Syrian Rue acts like an MAO inhibitor, and combining MAOIs with dextromethorphan can cause a dangerous reaction called serotonin syndrome (agitation, sweating, fast heartbeat, high fever). This is mostly theory-based, but I'd play it safe and check with your pharmacist or doctor before mixing the two.

Contact Dr. Brian directly if you have additional questions on this interaction
References
  1. Zhao, T., He, Y. Q., Wang, J., Ding, K. M., Wang, C. H., and Wang, Z. T. Inhibition of human cytochrome P450 enzymes 3A4 and 2D6 by β-carboline alkaloids, harmine derivatives. Phytother Res 2011;25(11):1671-1677.

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

Syrian Rue Also Interacts With Other Medications

Syrian Rue has 1,121 moderate known drug interactions in our database — the Dextromethorphan HBr, Guaifenesin one above is among them. If you take anything else, check it against the full list below.

Moderate 1,121 No known interaction 1,709 (incl. Dextromethorphan HBr, Guaifenesin)
Taking anything else?

Check Another of Your Medications Against Syrian Rue

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

Everything else you take

See Every Supplement That Interacts With Dextromethorphan HBr, Guaifenesin

This page covers Syrian Rue only. If you take other supplements — a multivitamin, a sleep aid, something for joints — the Dextromethorphan HBr, Guaifenesin page lists 171 supplements it has a documented interaction with, worst first, each one explained.

The most serious there right now: Activated Charcoal (major), Grapefruit (major), Lithium (major), St. John's Wort (major) and 1,3-dmaa (moderate).

By severity: 4 major · 125 moderate · 42 minor.

From our pharmacist’s overview on the Dextromethorphan HBr, Guaifenesin page

“A bad cough tends to hit when supplements are already on the counter, and anyone reaching for Vicks Formula 44E (dextromethorphan HBr and guaifenesin) should treat this list with real care, since 4 of its 171 entries carry a major rating. Two themes dominate.”

Reviewed by 2 · Jul 18, 2026

Open the Dextromethorphan HBr, Guaifenesin interaction page

How much people look this up

Where Syrian Rue and Dextromethorphan HBr, Guaifenesin 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 Syrian Rue
Steady interest

#1,039 of 1,452 supplements looked up here in the last 7 days

29 days ago today
Medication Dextromethorphan HBr, Guaifenesin
Top half

#1,382 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 Syrian Rue

What is Syrian Rue?

  • Traditional remedy for digestive complaints
  • Used as a sedative or for mood
  • Antimicrobial and antiparasitic folk use
  • Used as a dye and incense
  • Recreational psychoactive use

Syrian Rue (Peganum harmala) is a traditional plant whose seeds contain potent compounds called harmala alkaloids, including MAO-inhibiting harmine and harmaline. There is little reliable human evidence that it safely treats any condition, and it carries real risks of toxicity, so it is not recommended for self-treatment. Talk to a pharmacist or doctor before considering it.

Read the full Syrian Rue monograph & all interactions
Keep checking

Other Drugs & Syrian Rue

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

0Major
1,121Moderate
0Minor
1,708No known interaction
1,121 Moderate 1,708 No interaction
Moderate

1,121 interactions

Showing 60 of 1,121 moderate interactions — see all 1,121 on the Syrian Rue page.

No interaction 1,708 drugs checked — show the list

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

Showing the 60 most looked-up of 1,708 medications checked with no known interaction — see the full Syrian Rue list.

See the full Syrian Rue interactions page
See every supplement checked against Dextromethorphan HBr, Guaifenesin, 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
Syrian Rue monograph › Uses, safety, and every drug interaction in one place Dextromethorphan HBr, Guaifenesin supplement interactions › Every supplement checked against Dextromethorphan HBr, Guaifenesin, ranked by severity 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 Syrian Rue and Dextromethorphan HBr, Guaifenesin Together: Common Questions

Can you take Syrian Rue with Dextromethorphan HBr, Guaifenesin?
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.
Why are there 3 interactions listed for Syrian Rue and Dextromethorphan HBr, Guaifenesin?
Dextromethorphan HBr, Guaifenesin belongs to more than one drug category, and Syrian Rue interacts with each of those categories a little differently. We list every one so nothing is missed — your overall risk is driven by the most serious of them.
How serious is the Syrian Rue and Dextromethorphan HBr, Guaifenesin 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

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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.