Herb & supplement monograph

Thiamine Drug Interactions, Uses, Effectiveness, Safety & More

Medically reviewed by Dr. Brian Staiger, PharmD, BCPS ·Updated July 2026

What is this page for?

First and foremost: how Thiamine interacts with medications. The heart of this page is the interaction list — every drug Thiamine is known to interact with, and how serious each one is.

But these pages have grown well beyond that into a full monograph — what Thiamine is, what people use it for and how strong the evidence is, its safety and side effects, and answers to the questions we’re asked most — written and reviewed by the clinical staff at HelloPharmacist. It’s educational information from our licensed clinical databases, not medical advice, and we don’t sell or endorse products. Our editorial policy

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Reviewed by licensed pharmacists Sourced from Natural Medicines

On this page

Thiamine Drug Interactions: The Bottom Line

From the HelloPharmacist Editorial Team · Updated July 2026

Based on the available evidence, the overall risk of a clinically significant Thiamine drug interaction appears low for most people. Only a handful of medications interact with thiamine at all, none is rated major or moderate, and every listed interaction is minor.

Interactions deserving the most attention

The one combination worth knowing about involves the antibiotic trimethoprim. Human and laboratory research suggests it blocks several transporters the body uses to move thiamine in the gut, liver, and kidneys, which can actually raise thiamine levels in the blood rather than lower them. Because the effect is graded minor and thiamine is a water-soluble vitamin with a wide safety margin, this is more of an interesting finding than a practical safety concern for most people.

What the rest of the list means

Thiamine's interaction profile is unusually short and quiet compared with many supplements. The concerns that do exist come from possible effects on transporter proteins that shuttle the vitamin around the body, and none of them has been tied to meaningful harm in people. In practical terms, thiamine fits comfortably alongside most medication regimens.

Check your medications against Thiamine

Based on HelloPharmacist’s Thiamine interaction data and reviewed under our editorial standards.

Interaction report

Drugs that interact with Thiamine

3 medications have a known interaction with Thiamine, graded by severity. Select any drug for the full evidence-based detail.

Don’t want to scroll the list? Just ask.

Tell us the medications you take and we’ll check each one against Thiamine using our pharmacist-reviewed interaction data.

AI summaries are generated from our Thiamine interaction data for education only — always confirm with your pharmacist. How we use AI

2,830 drugs

Read The List Like a Pharmacist

What Severity, Likelihood & Evidence Mean

Severity — How Serious It Can Be

Major. Clinically significant; generally best avoided, or used only under direct professional supervision.
Moderate. May need monitoring, a dose adjustment, or separating the times you take each one.
Minor. Generally not clinically significant, but still worth noting and mentioning to your pharmacist.
No known interaction. Checked against our sources with nothing documented — not the same as proven safety.

Likelihood — How Well It’s Documented

Likely. Well-controlled human studies have demonstrated the likely existence of this interaction
Probable. 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.
Possible. 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
Unlikely. Interaction has been demonstrated in animal or in lab research but has been shown not to occur in humans.

Where This Data Comes From

Interaction records are evidence-graded and sourced from the Natural Medicines database (TRC Healthcare), the same reference used by pharmacists and hospitals.
Each drug listed above links to the full report for that exact Thiamine combination — clinical detail, likelihood, evidence level, and citations.
Content is reviewed by licensed HelloPharmacist pharmacists — see our data sources and editorial standards.

The big picture

The kinds of drugs Thiamine affects

Every type of medication (drug category) Thiamine is known to interact with. Open any category for the detail — or search your exact drug in the checker above.

All 1 drug category Thiamine interacts with

Trimethoprim (proloprim)

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.

Likelihood Probable Evidence B

Monograph

Thiamine: Uses, Safety & Side Effects

Medically reviewed by Dr. Brian Staiger, PharmD, BCPS · Reviewed July 2026 ·

The bottom line

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.

Common forms: Tablets, capsules, oral liquids, injectable solutions (prescription), part of B-complex and multivitamins

People commonly use it for

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

Popular and traditional uses — not proof it works. See “Uses & effectiveness” below for the evidence.

Safety at a glance

OverallLikely safe

Thiamine is generally very safe when taken by mouth in normal amounts.

PregnancyLikely Safe

When used orally in dietary amounts of 1.4 mg daily. A tolerable upper intake level (UL) has not been established for healthy individuals.

Read the full pregnancy detail

BreastfeedingLikely Safe

When used orally in dietary amounts of 1.4 mg daily. A tolerable upper intake level (UL) has not been established for healthy individuals.

Read the full breastfeeding detail

Pregnancy & breastfeeding ratings are from Natural Medicines (Therapeutic Research Center). Safety guidance is general; always confirm with your pharmacist or doctor for your situation.

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Overview

Thiamine, also called vitamin B1, is one of the eight B vitamins. It is an essential nutrient, which means your body cannot make it and you must get it from food or supplements.

Thiamine is found naturally in many foods, including whole grains, beans, lentils, pork, nuts, seeds, and fortified breads and cereals. In many countries, grain products are enriched with thiamine to help prevent deficiency.

People take thiamine supplements to prevent or treat a shortage of the vitamin, as part of a B-complex or multivitamin, or for specific medical conditions diagnosed by a doctor. It comes as oral tablets and capsules, and as an injection used in hospitals.

How it works

Inside the body, thiamine is converted into an active form called thiamine pyrophosphate. This acts as a helper (a coenzyme) for several key enzymes that break down carbohydrates and turn food into usable energy.

Because of this role, thiamine is especially important for tissues that use a lot of energy, such as the brain, nerves, and heart. When thiamine is low, these systems are the first to show problems.

Thiamine is a water-soluble vitamin, so the body does not store large amounts of it. Any extra is mostly removed in the urine, which is part of why it has a strong safety record.

Effectiveness

Does Thiamine work?

Here’s what the research actually shows for each use — graded by Natural Medicines and reviewed by Dr. Brian Staiger, PharmD.

Evidence overview · 38 uses evaluated

2 Effective 1 Possibly effective 4 Leans ineffective 31 Insufficient evidence

How to read these evidence grades

Natural Medicines’ 7-point scale. We show each rating’s label word-for-word.

1EffectiveStrong, consistent evidence it works.

2Likely EffectiveGood evidence, though not yet conclusive.

3Possibly EffectiveSome evidence suggests a benefit.

4Possibly IneffectiveSome evidence it may not help.

5Likely IneffectiveFairly strong evidence it doesn’t help.

6IneffectiveStrong evidence it doesn’t work.

7Insufficient Reliable Evidence to RateToo little research to say either way.

Effective Thiamine deficiency