Drug & supplement interaction

Grapefruit + Ephedrine, Phenobarbital, Potassium Iodide, Theophylline

Ephedrine, Phenobarbital, Potassium Iodide, Theophylline brand names: Quadrinal, Mudrane.

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

Grapefruit has a moderate interaction with Ephedrine, Phenobarbital, Potassium Iodide, Theophylline. Grapefruit juice can decrease blood levels of theophylline, potentially decreasing the effectiveness of theophylline.

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

Grapefruit and Ephedrine, Phenobarbital, Potassium Iodide, Theophylline

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

Moderate

THEOPHYLLINE

Why this applies to Ephedrine, Phenobarbital, Potassium Iodide, Theophylline

Ephedrine, Phenobarbital, Potassium Iodide, Theophylline falls under THEOPHYLLINE — the group Grapefruit interacts with.

Grapefruit juice can decrease blood levels of theophylline, potentially decreasing the effectiveness of theophylline.
Clinical research shows that grapefruit juice seems to modestly decrease theophylline levels when given concurrently with sustained-release theophylline. The mechanism of this interaction is unknown.
Severity Moderate Likelihood Probable Evidence B

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

This combination product contains theophylline, a breathing medicine that has to stay within a fairly narrow range to work well without causing trouble. The thing to know here is that grapefruit juice can actually lower your theophylline levels a bit. The exact reason isn't fully understood, but studies using sustained-release theophylline showed a modest drop when grapefruit was added.

In plain terms, this isn't about a dangerous buildup. It's the opposite: too little theophylline may mean your asthma or breathing symptoms aren't as well controlled as they should be. The effect appears mild and the evidence is on the weaker side, so don't panic if you've had grapefruit. Just talk with your pharmacist or doctor before making it a regular part of your routine.

Contact Dr. Brian directly if you have additional questions on this interaction
References
  1. Gupta MC, Garg SK, Badyal D, et al. Effect of grapefruit juice on the pharmacokinetics of theophylline in healthy male volunteers. Methods Find Exp Clin Pharmacol 1999;21:679-82. 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

Grapefruit Also Interacts With Other Medications

Grapefruit has 776 major, 150 moderate, 63 minor known drug interactions in our database — the Ephedrine, Phenobarbital, Potassium Iodide, Theophylline one above is one of them. If you take anything else, check it against the full list below.

Major 776 Moderate 150 Minor 63 No known interaction 1,841 (incl. Ephedrine, Phenobarbital, Potassium Iodide, Theophylline)
Taking anything else?

Check Another of Your Medications Against Grapefruit

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

From our Q&A

Questions About Grapefruit or Ephedrine, Phenobarbital, Potassium Iodide, Theophylline

These are real questions from readers, answered by the pharmacists at HelloPharmacist — written by a person, not generated from a database. Each one covers Grapefruit or Ephedrine, Phenobarbital, Potassium Iodide, Theophylline, 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 Ephedrine, Phenobarbital, Potassium Iodide, Theophylline

This page covers Grapefruit only. If you take other supplements — a multivitamin, a sleep aid, something for joints — the Ephedrine, Phenobarbital, Potassium Iodide, Theophylline page lists 179 supplements it has a documented interaction with, worst first, each one explained.

The most serious there right now: 1,4-butanediol (major), Activated Charcoal (major), Beer (major), Black Tea (major) and Caffeine (major).

By severity: 22 major · 136 moderate · 21 minor.

From our pharmacist’s overview on the Ephedrine, Phenobarbital, Potassium Iodide, Theophylline page

“Quadrinal and Mudrane combine a stimulant (ephedrine) and a sedative (phenobarbital) with theophylline and potassium iodide, so supplements that push in either direction can cause real trouble, and this list of 179 interacting ingredients deserves genuine care. The everyday risk to understand is caffeine: with ephedrine it has been lin…”

Reviewed by 2 · Jul 18, 2026

Open the Ephedrine, Phenobarbital, Potassium Iodide, Theophylline interaction page

One more smart check

Taking Grapefruit as Part of a Dietary Supplement Product? Check It for Interactions

We hold 1,144 real supplement products containing Grapefruit, each one broken down to its full ingredient list — and combination products often carry several ingredients besides Grapefruit. Your product may carry other ingredients that interact with Ephedrine, Phenobarbital, Potassium Iodide, Theophylline too, on top of Grapefruit — 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 Grapefruit and Ephedrine, Phenobarbital, Potassium Iodide, Theophylline 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 Grapefruit
Top 10%

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

29 days ago today
Medication Ephedrine, Phenobarbital, Potassium Iodide, Theophylline
Top half

#1,016 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 Grapefruit

What is Grapefruit?

Grapefruit
  • Source of vitamin C
  • Digestive complaints
  • Antimicrobial (seed extract)
  • Weight management
  • Skin and cosmetic uses

Grapefruit is a nutritious citrus fruit rich in vitamin C and other nutrients, and it is generally safe to eat. However, grapefruit is famous for serious interactions with many prescription medicines, so anyone taking medication should check with a pharmacist or doctor before consuming it regularly.

Read the full Grapefruit monograph & all interactions
Keep checking

Other Drugs & Grapefruit

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

776Major
150Moderate
63Minor
1,840No known interaction
776 Major 150 Moderate 63 Minor 1,840 No interaction
Major

776 interactions

Showing 60 of 776 major interactions — see all 776 on the Grapefruit page.

Moderate

150 interactions

Showing 60 of 150 moderate interactions — see all 150 on the Grapefruit page.

Minor

63 interactions

Showing 60 of 63 minor interactions — see all 63 on the Grapefruit page.

No interaction 1,840 drugs checked — show the list

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

Showing the 60 most looked-up of 1,840 medications checked with no known interaction — see the full Grapefruit list.

See the full Grapefruit interactions page
See every supplement checked against Ephedrine, Phenobarbital, Potassium Iodide, Theophylline, 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
Grapefruit monograph › Uses, safety, and every drug interaction in one place Ephedrine, Phenobarbital, Potassium Iodide, Theophylline supplement interactions › Every supplement checked against Ephedrine, Phenobarbital, Potassium Iodide, Theophylline, 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 Grapefruit and Ephedrine, Phenobarbital, Potassium Iodide, Theophylline Together: Common Questions

Can you take Grapefruit with Ephedrine, Phenobarbital, Potassium Iodide, Theophylline?
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 Grapefruit and Ephedrine, Phenobarbital, Potassium Iodide, Theophylline 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 Ephedrine, Phenobarbital, Potassium Iodide, Theophylline and Grapefruit?

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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.