Drug & supplement interaction

Thiamine + Trimethoprim

Trimethoprim brand names: Proloprim, Trimpex.

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

Thiamine has a minor interaction with Trimethoprim. Trimethoprim might increase blood levels of thiamine.

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

Thiamine and Trimethoprim

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

Minor

TRIMETHOPRIM (Proloprim)

Trimethoprim might increase blood levels of thiamine.
In vitro, animal, and clinical research suggest that trimethoprim inhibits intestinal thiamine transporter ThTR-2, hepatic transporter OCT1, and renal transporters OCT2, MATE1, and MATE2, resulting in paradoxically increased thiamine plasma concentrations.
Severity Minor 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 a slightly surprising one. When you take trimethoprim alongside thiamine (vitamin B1), the antibiotic can nudge your thiamine blood levels up rather than down. That's because trimethoprim blocks several of the transporters your body uses to move thiamine in and out of cells and to clear it through the kidneys, so a bit more thiamine ends up hanging around in your bloodstream.

The good news: this is rated a minor interaction, and thiamine is a water-soluble vitamin, so extra amounts are usually flushed out in your urine and well tolerated. The evidence is fairly modest, and there's no reason to expect harm at normal doses. Still, if you take thiamine regularly or are on it for a medical reason, it's worth a quick mention to your pharmacist or doctor.

Contact Dr. Brian directly if you have additional questions on this interaction
References
  1. Vora B, Wen A, Yee SW, et al. The Effect of Trimethoprim on Thiamine Absorption: A Transporter-Mediated Drug-Nutrient Interaction. Clin Pharmacol Ther 2023;114(2):381-392.

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

Thiamine Also Interacts With Other Medications

Thiamine has 2 minor 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.

Minor 2 No known interaction 2,828 (incl. Trimethoprim)
Taking anything else?

Check Another of Your Medications Against Thiamine

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

From our Q&A

Questions About Thiamine 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 Thiamine 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 Thiamine 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), Arsenic (major), Cesium (major) and Ephedra (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 Thiamine as Part of a Dietary Supplement Product? Check It for Interactions

We hold 3 real supplement products containing Thiamine, each one broken down to its full ingredient list — and combination products often carry several ingredients besides Thiamine. Your product may carry other ingredients that interact with Trimethoprim too, on top of Thiamine — 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 Thiamine 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 Thiamine
Top 25%

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

29 days ago today
Medication Trimethoprim
Steady interest

#1,470 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 Thiamine

What is Thiamine?

  • Preventing or treating thiamine deficiency
  • Beriberi
  • Wernicke-Korsakoff syndrome (alcohol-related)
  • Supporting energy and nerve function
  • General B-vitamin supplementation

Thiamine (vitamin B1) is an essential nutrient your body needs to turn food into energy and to keep your nerves and heart healthy. Most people get enough from food, but supplements are clearly useful for deficiency, certain medical conditions, and people with heavy alcohol use. Talk with your pharmacist or doctor before starting it, especially if you have a chronic illness.

Read the full Thiamine 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 & Thiamine

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

0Major
0Moderate
2Minor
2,827No known interaction
2 Minor 2,827 No interaction
Minor

2 interactions

No interaction 2,827 drugs checked — show the list

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

Showing the 60 most looked-up of 2,827 medications checked with no known interaction — see the full Thiamine list.

See the full Thiamine 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
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Pharmacist Counseling Corner

Taking Thiamine and Trimethoprim Together: Common Questions

Can you take Thiamine with Trimethoprim?
There is only a minor interaction, which is usually not serious. Read the full details on this page and confirm with your pharmacist before combining them.
How serious is the Thiamine and Trimethoprim interaction?
We rate it Minor. This interaction is generally not clinically significant, but it remains worth noting and discussing with your pharmacist.

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

Still Have a Question About Trimethoprim and Thiamine?

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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.