Drug & supplement interaction

5-htp + Dextromethorphan HBr, Phenylephrine HCl

Dextromethorphan HBr, Phenylephrine HCl brand name: Vicks Formula 44D Cough & Head Congestion Relief.

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

5-htp has a moderate interaction with Dextromethorphan HBr, Phenylephrine HCl. Combining serotonergic drugs with 5-HTP might cause additive serotonergic effects.

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

5-htp and Dextromethorphan HBr, Phenylephrine HCl

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

Moderate

SEROTONERGIC DRUGS

Why this applies to Dextromethorphan HBr, Phenylephrine HCl

Dextromethorphan HBr, Phenylephrine HCl falls under SEROTONERGIC DRUGS — the group 5-htp interacts with.

Combining serotonergic drugs with 5-HTP might cause additive serotonergic effects.
5-HTP can increase serotonin levels and cause serotonergic effects. Theoretically, combining serotonergic drugs with 5-HTP might increase the risk of serotonergic side effects, including serotonin syndrome and cerebral vasoconstrictive disorders. However, serotonin syndrome with 5-HTP has not yet been reported in humans. Monitor patients for signs of serotonin syndrome and other serotonergic side effects if using 5-HTP 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

Here's the thing to know about this combo: the dextromethorphan in your Vicks Formula 44D nudges serotonin upward in the brain, and 5-HTP does the same by feeding your body more raw material to make serotonin. Stack the two together and you could, in theory, push serotonin a little too high. That's what we mean by additive serotonergic effects.

If serotonin climbs too far, you might notice agitation, shivering, a racing heart, sweating, muscle twitching, or feeling confused. I'll be honest with you though: this is a theoretical caution based on how these substances work, and serotonin syndrome from 5-HTP hasn't actually been reported in people. The phenylephrine part isn't the concern here. For a short cough or cold, the risk is low, but check with your pharmacist before pairing them.

Contact Dr. Brian directly if you have additional questions on this interaction
References
  1. Birdsall TC. 5-Hydroxytryptophan: A Clinically-Effective Serotonin Precursor. Altern Med Rev 1998;3:271-80.
  2. Singhal AB, Caviness VS, Begleiter AF, et al. Cerebral vasoconstriction and stroke after use of serotonergic drugs. Neurology 2002;58:130-3. PubMed
  3. Maffei ME. 5-Hydroxytryptophan (5-HTP): Natural Occurrence, Analysis, Biosynthesis, Biotechnology, Physiology and Toxicology. Int J Mol Sci. 2020;22(1):181. 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

5-htp Also Interacts With Other Medications

5-htp has 397 moderate known drug interactions in our database — the Dextromethorphan HBr, Phenylephrine HCl one above is one of them. If you take anything else, check it against the full list below.

Moderate 397 No known interaction 2,433 (incl. Dextromethorphan HBr, Phenylephrine HCl)
Taking anything else?

Check Another of Your Medications Against 5-htp

Covers the 2,830 medications we’ve checked against 5-htp. Pick one to open its full report.

From our Q&A

Questions About 5-htp or Dextromethorphan HBr, Phenylephrine HCl

These are real questions from readers, answered by the pharmacists at HelloPharmacist — written by a person, not generated from a database. Each one covers 5-htp or Dextromethorphan HBr, Phenylephrine HCl, not necessarily both, but if one is close to what you were about to ask it will usually go further than a database entry can:

Not quite your question? Ask our pharmacists directly — it is free, there is no account required, and answers usually come back within 24 hours.

Everything else you take

See Every Supplement That Interacts With Dextromethorphan HBr, Phenylephrine HCl

This page covers 5-htp only. If you take other supplements — a multivitamin, a sleep aid, something for joints — the Dextromethorphan HBr, Phenylephrine HCl page lists 197 supplements it has a documented interaction with, worst first, each one explained.

The most serious there right now: Activated Charcoal (major), Ephedra (major), Grapefruit (major), Lithium (major) and Peyote (major).

By severity: 8 major · 148 moderate · 41 minor.

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

“The supplement interactions with Dextromethorphan HBr and Phenylephrine HCl (Vicks Formula 44D) deserve real respect, and the small cluster of major-rated ones should be understood before you reach for the bottle. Phenylephrine is a stimulant-type decongestant, so anything with caffeine (coffee, tea, cola nut) or ephedrine-like herbs s…”

Reviewed by 2 · Jul 18, 2026

Open the Dextromethorphan HBr, Phenylephrine HCl interaction page

One more smart check

Taking 5-htp as Part of a Dietary Supplement Product? Check It for Interactions

We hold 1,651 real supplement products containing 5-htp, each one broken down to its full ingredient list — and combination products often carry several ingredients besides 5-htp. Your product may carry other ingredients that interact with Dextromethorphan HBr, Phenylephrine HCl too, on top of 5-htp — and those would not show on this page. Find your product and we’ll check every ingredient on its label:

How much people look this up

Where 5-htp and Dextromethorphan HBr, Phenylephrine HCl 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 5-htp
Top 10%

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

29 days ago today
Medication Dextromethorphan HBr, Phenylephrine HCl
Steady interest

#2,537 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 5-htp

What is 5-htp?

5-htp
  • Low mood or depression
  • Anxiety and stress
  • Sleep problems
  • Migraine headaches
  • Fibromyalgia
  • Appetite or weight management

5-HTP is a compound your body uses to make serotonin, and people take it as a supplement hoping to improve mood, sleep, and headaches. Some early research is promising, but the overall evidence is limited and not conclusive. Because it can affect serotonin and may interact with medicines, talk to your pharmacist or doctor before using it.

Read the full 5-htp monograph & all interactions
Keep checking

Other Drugs & 5-htp

Browse every other medication we’ve checked against 5-htp — both the ones with a known interaction and the ones without. Select any drug for the full detail.

0Major
397Moderate
0Minor
2,432No known interaction
397 Moderate 2,432 No interaction
Moderate

397 interactions

Showing 60 of 397 moderate interactions — see all 397 on the 5-htp page.

No interaction 2,432 drugs checked — show the list

We also checked 5-htp against these medications and found no known interaction in our sources. Open any page to confirm.

Showing the 60 most looked-up of 2,432 medications checked with no known interaction — see the full 5-htp list.

See the full 5-htp interactions page
See every supplement checked against Dextromethorphan HBr, Phenylephrine HCl, 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
5-htp monograph › Uses, safety, and every drug interaction in one place Dextromethorphan HBr, Phenylephrine HCl supplement interactions › Every supplement checked against Dextromethorphan HBr, Phenylephrine HCl, 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 5-htp and Dextromethorphan HBr, Phenylephrine HCl Together: Common Questions

Can you take 5-htp with Dextromethorphan HBr, Phenylephrine HCl?
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 5-htp and Dextromethorphan HBr, Phenylephrine HCl 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 Dextromethorphan HBr, Phenylephrine HCl and 5-htp?

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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.