Drug & supplement interaction

Arsenic + Trimethoprim

Trimethoprim brand names: Proloprim, Trimpex.

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

Arsenic has a major interaction with Trimethoprim. Arsenic trioxide can prolong the QT interval.

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

Arsenic and Trimethoprim

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

Major

QT INTERVAL-PROLONGING DRUGS

Why this applies to Trimethoprim

Trimethoprim falls under QT INTERVAL-PROLONGING DRUGS — the group Arsenic interacts with.

Arsenic trioxide can prolong the QT interval.
Up to 40% of patients treated with prescription arsenic trioxide have a prolonged QT interval on their electrocardiogram (ECG). Theoretically, non-prescription arsenic could have an additive effect when combined with drugs that prolong the QT interval.
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 concern with mixing arsenic and trimethoprim. Both can affect your heart's electrical timing, something we call the QT interval. Prescription arsenic (arsenic trioxide) is well known for stretching out that interval, and trimethoprim can nudge it in the same direction too. Put them together and the effects may stack up, which in rare cases can lead to a dangerous irregular heartbeat.

The good news is this is mostly a worry with concentrated arsenic, not the tiny amounts in food. Since there's no real reason to take arsenic on purpose, the simplest move is to avoid arsenic-containing products entirely. If you have any heart rhythm history or take other QT-affecting medicines, please check in with your pharmacist or doctor first.

Contact Dr. Brian directly if you have additional questions on this interaction
References
  1. McEvoy GK, ed. AHFS Drug Information. Bethesda, MD: American Society of Health-System Pharmacists, 1998.

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

Arsenic Also Interacts With Other Medications

Arsenic has 160 major, known drug interactions in our database — the Trimethoprim one above is one of them. If you take anything else, check it against the full list below.

Major 160 No known interaction 2,670 (incl. Trimethoprim)
Taking anything else?

Check Another of Your Medications Against Arsenic

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

From our Q&A

Questions About Arsenic or Trimethoprim

These are real questions from readers, answered by the pharmacists at HelloPharmacist — written by a person, not generated from a database. Each one covers Arsenic or Trimethoprim, 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 Trimethoprim

This page covers Arsenic only. If you take other supplements — a multivitamin, a sleep aid, something for joints — the Trimethoprim 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), Cesium (major), Ephedra (major) and Grapefruit (major).

By severity: 7 major · 41 moderate · 11 minor.

From our pharmacist’s overview on the Trimethoprim page

“Most prescriptions for trimethoprim are short courses for bladder infections, which makes it easy to assume nothing swallowed alongside it matters much. Seven of the 59 supplement and herbal ingredients here rate major, though, and nearly all of them involve heart rhythm: grapefruit in large amounts, ephedra, and a few others can prolo…”

Reviewed by 2 · Jul 18, 2026

Open the Trimethoprim interaction page

One more smart check

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

We hold 4 real supplement products containing Arsenic, each one broken down to its full ingredient list — and combination products often carry several ingredients besides Arsenic. Your product may carry other ingredients that interact with Trimethoprim too, on top of Arsenic — 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 Arsenic and Trimethoprim 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 Arsenic
Steady interest

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

29 days ago today
Medication Trimethoprim
Steady interest

#1,454 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 Arsenic

What is Arsenic?

Arsenic
  • Homeopathic digestive complaints
  • Traditional remedy for inflammatory conditions
  • Prescription use in certain leukemias (medical, not supplement)
  • Anxiety and restlessness (homeopathic tradition)

Arsenic is a naturally occurring element that is toxic in its inorganic form, and it is not a supplement you should take to improve your health. While arsenic trioxide is a real, doctor-prescribed cancer medicine, and tiny homeopathic doses exist, there is no good reason to take arsenic on your own. Avoid arsenic-containing products and talk to a pharmacist or doctor about any concerns.

Read the full Arsenic monograph & all interactions
Medication

About Trimethoprim

What Is Trimethoprim Used For?

Brand names: Proloprim, Trimpex

Trimethoprim tablets are used to treat initial episodes of uncomplicated urinary tract infections (UTIs). It works against specific bacteria — including E. coli, Proteus mirabilis, Klebsiella pneumoniae, Enterobacter species, and coagulase-negative Staphylococcus species (including S. saprophyticus).

It should only be used when there is a proven or strongly suspected bacterial infection. Your doctor may order a urine culture to confirm which bacteria are present and whether trimethoprim will work against them.

See full Trimethoprim drug information
Keep checking

Other Drugs & Arsenic

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

160Major
0Moderate
0Minor
2,669No known interaction
160 Major 2,669 No interaction
Major

160 interactions

Showing 60 of 160 major interactions — see all 160 on the Arsenic page.

No interaction 2,669 drugs checked — show the list

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

Showing the 60 most looked-up of 2,669 medications checked with no known interaction — see the full Arsenic list.

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

Can you take Arsenic with Trimethoprim?
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 Arsenic and Trimethoprim 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 Trimethoprim and Arsenic?

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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.