Drug & supplement interaction

Grapefruit + Procainamide

Procainamide brand names: Pronestyl, Procanbid, Procan SR, Procan, Pronestyl-SR.

Major Interaction
Medically reviewed by · Updated October 2026
The bottom line Major

Grapefruit has a major interaction with Procainamide. Grapefruit or grapefruit juice, especially if consumed in large amounts, can cause additive QT interval prolongation when taken with QT interval-prolonging drugs, potentially increasing the risk of ventricular arrhythmias.

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

Grapefruit and Procainamide

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

Major

QT INTERVAL-PROLONGING DRUGS

Why this applies to Procainamide

Procainamide falls under QT INTERVAL-PROLONGING DRUGS — the group Grapefruit interacts with.

Grapefruit or grapefruit juice, especially if consumed in large amounts, can cause additive QT interval prolongation when taken with QT interval-prolonging drugs, potentially increasing the risk of ventricular arrhythmias.
Clinical research in healthy volunteers shows that drinking 6 liters of grapefruit juice over 6 hours prolonged the QTc by a peak amount of 14 milliseconds (ms). This prolongation was similar to the QT prolongation caused by the drug moxifloxacin. In individuals with long QT syndrome, a smaller dose of grapefruit juice, 1.5 liters, resulted in a greater peak QTc prolongation of about 30 ms. The effect of smaller quantities of grapefruit juice on the QT interval is unclear.
Severity Major 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

Here's the thing about procainamide: one of its known effects is stretching out the heart's electrical reset time, what we call the QT interval. Grapefruit can do a little of the same thing on its own. Put them together and those effects can add up, which in rare cases raises the risk of a dangerous heart rhythm. Procainamide is one of those medicines where small shifts deserve real attention.

The catch is the dose. The studies that showed a clear effect used huge amounts, several liters of grapefruit juice in a few hours. A normal piece of fruit or a regular glass probably won't matter much. Still, because procainamide already affects your rhythm, I'd run your grapefruit habits by your pharmacist or doctor first.

Contact Dr. Brian directly if you have additional questions on this interaction
References
  1. Chorin E, Hochstadt A, Granot Y, et al. Grapefruit juice prolongs the QT interval of healthy volunteers and patients with long QT syndrome. Heart Rhythm. 2019. pii: S1547-5271(19)30368-6. 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 777 major, 151 moderate, 62 minor known drug interactions in our database — the Procainamide one above is one of them. If you take anything else, check it against the full list below.

Major 777 Moderate 151 Minor 62 No known interaction 1,840 (incl. Procainamide)
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 Procainamide

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 Procainamide, 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 Procainamide

This page covers Grapefruit only. If you take other supplements — a multivitamin, a sleep aid, something for joints — the Procainamide page lists 59 supplements it has a documented interaction with, worst first, each one explained.

The most serious there right now: Activated Charcoal (major), American Hellebore (major), Arsenic (major), Cesium (major) and Ephedra (major).

By severity: 9 major · 39 moderate · 11 minor.

From our pharmacist’s overview on the Procainamide page

“Procainamide treats serious heart rhythm problems, and anything that nudges the heart's electrical timing further can turn a helpful drug into a hazard, so I would give this list genuine care. Nine of the 59 supplements on file carry a major rating, and the 13 QT prolongers are the ones to understand first.”

Reviewed by 2 · Jul 18, 2026

Open the Procainamide 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 Procainamide 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 Procainamide 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%

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

29 days ago today
Medication Procainamide
Top half

#1,098 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
Medication

About Procainamide

What Is Procainamide Used For?

Brand names: Pronestyl, Procanbid, Procan SR, Procan, Pronestyl-SR

Procainamide hydrochloride injection treats documented ventricular arrhythmias (abnormal heart rhythms starting in the lower heart chambers) that your doctor judges to be life-threatening. One example is sustained ventricular tachycardia, a fast, dangerous heartbeat.

It is generally not recommended for milder rhythm problems. It should be avoided for asymptomatic ventricular premature contractions (extra beats that cause no symptoms). The same use is listed for the Procainamide Hydrochloride, Procainamide HCl and Procainamide Hci injection products.

See full Procainamide drug information
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.

777Major
151Moderate
62Minor
1,839No known interaction
777 Major 151 Moderate 62 Minor 1,839 No interaction
Major

777 interactions

Showing 60 of 777 major interactions — see all 777 on the Grapefruit page.

Moderate

151 interactions

Showing 60 of 151 moderate interactions — see all 151 on the Grapefruit page.

Minor

62 interactions

Showing 60 of 62 minor interactions — see all 62 on the Grapefruit page.

No interaction 1,839 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 1,839 medications checked with no known interaction — see the full Grapefruit list.

See the full Grapefruit interactions page
See every supplement checked against Procainamide, 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 Procainamide supplement interactions › Every supplement checked against Procainamide, ranked by severity Procainamide 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 Grapefruit and Procainamide Together: Common Questions

Can you take Grapefruit with Procainamide?
This is rated a major interaction, so the combination is best avoided unless your doctor or pharmacist has specifically told you otherwise. Read the full details on this page and confirm with your pharmacist before combining them.
How serious is the Grapefruit and Procainamide interaction?
We rate it Major. This is a clinically significant combination that is generally best avoided, or used only under the direct supervision of your physician or pharmacist.

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

Still Have a Question About Procainamide 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.