Benfotiamine Side Effects: What Studies Report

Benfotiamine has generally been well tolerated in clinical trials. In placebo-controlled studies using 300 to 600 mg a day for up to 24 months, side effects were no more common with benfotiamine than with placebo, and none were serious. What is not known is how safe it is over many years or at higher doses, because no trial has followed people that long.

Key Takeaways

Benfotiamine is a synthetic, fat-soluble form of vitamin B1 (thiamin) that the body converts to thiamin.
In trials of 300 to 600 mg a day lasting 3 weeks to 24 months, adverse event rates were similar to placebo.
In a phase I study of healthy adults, the drug-related findings were small changes in lab tests (a rise in the liver enzyme ALT and white cells in urine), all mild or moderate.
No upper limit has been set for thiamin, but long-term safety of high-dose benfotiamine beyond two years has not been studied.
Talk to a clinician first if you are pregnant or breastfeeding, take furosemide or fluorouracil, or have a health condition.

Evidence grade: Moderate. Several small randomized, placebo-controlled trials (about 40 to 165 people each, lasting 3 weeks to 24 months) consistently report good tolerability, but none followed people for longer than two years.

What Is Benfotiamine?

Benfotiamine is a synthetic derivative of thiamin (vitamin B1) used in some dietary supplements. Unlike the common supplement forms, thiamin mononitrate and thiamin hydrochloride, it is not water soluble. The body converts it to thiamin [1].

Thiamin itself is essential. Its active form, thiamin diphosphate, helps the body turn carbohydrates, fats and amino acids into energy [1]. Benfotiamine does not occur in food. It is sold on its own and in some combination products.

Forms and absorption

The main reason benfotiamine exists is absorption. In a study of 10 healthy young men, benfotiamine produced significantly higher thiamin levels in blood plasma and red cells than water-soluble thiamin mononitrate, even though the benfotiamine dose was only 40% as large [2]. In a later study of healthy volunteers, plasma thiamin availability from benfotiamine was about 11 times that from thiamin hydrochloride, and red-cell levels of the active form were about twice as high [3].

That matters for safety in one way: benfotiamine raises thiamin levels more than the same amount of ordinary thiamin. In a phase I study, thiamin and its active form built up moderately over a week of repeated dosing [4]. For amounts used in research and timing, see our guide to benfotiamine dosage.

Does Benfotiamine Have Side Effects?

It can, like any supplement, but trials have found few. Here is what the main placebo-controlled studies reported.

Study	Who and how long	Daily amount	What was reported on safety
Haupt 2005 [5]	40 adults, 3 weeks	400 mg (two 50 mg tablets four times a day)	No side effects attributable to benfotiamine
Stracke 2008 [6]	165 adults randomized, 6 weeks	300 mg or 600 mg	Well tolerated in all groups
Sheng 2021 [4]	Healthy adults, single doses and 10-day courses	150 to 1,200 mg single doses; up to 600 mg twice a day	Adverse events similar to placebo; all mild or moderate; most common drug-related findings were raised ALT and white cells in urine
Gibson 2020 [7]	70 adults with memory problems, 12 months	600 mg (300 mg twice a day)	No adverse events judged related to benfotiamine; dizziness 8% in both groups; digestive complaints 26% vs 34% on placebo
Ziegler 2026 [8]	57 adults, 12 months	600 mg (300 mg twice a day)	No relevant differences in adverse event rates versus placebo
Fraser 2012 [9]	67 adults, 24 months	300 mg	Blood thiamin rose markedly; no differences in the nerve and inflammation measures studied

Digestive effects

Online lists of benfotiamine side effects often include upset stomach, nausea or loose stools. In the 12-month trial that published a detailed symptom table, digestive problems were common in both groups but were not more frequent with benfotiamine: 26% of people taking it reported one, compared with 34% on placebo [7]. If benfotiamine bothers your stomach, taking it with a meal or lowering the amount is a reasonable step to discuss with your clinician.

Laboratory changes

In the phase I study, the drug-related findings most often recorded were a rise in ALT, a liver enzyme, and white blood cells in the urine. All adverse events were mild or moderate, no one had a serious event, and no one stopped early because of one [4]. The study was small and short, so it does not show whether these changes matter over time.

Dizziness and blood pressure

Dizziness and lower blood pressure are often listed online as benfotiamine side effects, but the trials here do not support a link. In the 12-month trial, dizziness occurred in 8% of people in both the benfotiamine and placebo groups [7]. Dizziness still deserves attention, whatever the cause, so mention it to your clinician.

Allergic reactions

In the 12-month trial, allergies were recorded only in the placebo group, not in people taking benfotiamine [7]. Still, any supplement can cause an allergic reaction. If you notice a rash, itching, swelling of the face or throat, or trouble breathing after taking benfotiamine, stop taking it and get medical help.

Is Fatigue a Side Effect of Benfotiamine?

Fatigue has not been reported as a side effect in the trials reviewed here [4][5][6][7][8]. Thiamin helps convert food into energy [1], but none of these trials tested benfotiamine as a way to reduce tiredness. If you feel unusually tired while taking it, remember that fatigue has many possible causes, and ongoing tiredness is worth raising with your doctor.

What We Do Not Know About Long-Term Use

The longest placebo-controlled trial lasted 24 months at 300 mg a day [9], and two trials lasted 12 months at 600 mg a day [7][8]. Nothing longer has been published. The Food and Nutrition Board did not set an upper limit for thiamin because high intakes from food or supplements (50 mg a day or more) have not produced reported adverse effects, partly because the body absorbs less and excretes the excess in urine [1]. That reasoning applies to ordinary thiamin. Benfotiamine is absorbed differently and raises blood levels further [2][3], so the absence of a thiamin upper limit should not be read as proof that any amount of benfotiamine is safe.

Some of the evidence also comes from people with health conditions rather than healthy adults, and several trials were small. The 2026 trial had co-authors employed by a benfotiamine manufacturer [8]. More, larger and longer studies are needed. For background on how B vitamins work in the body, see our article on the impact of B vitamins on the nervous system.

Safety and Interactions

Medications: NIH reports that thiamin is not known to interact with any medications, but some medicines lower thiamin levels. Furosemide, a loop diuretic, has been linked to thiamin loss in urine, and the chemotherapy drug fluorouracil has been linked to thiamin deficiency [1]. If you take either, talk to your doctor or pharmacist about your thiamin status before adding benfotiamine.
Other medicines: Few interaction studies exist for benfotiamine itself. Tell your doctor or pharmacist about every medicine and supplement you take.
Combining products: If you already take a B-complex or multivitamin, check the total thiamin and benfotiamine across your products with your pharmacist. Our B-complex dosage guide explains how B vitamins are usually combined.
Pregnancy and breastfeeding: The trials did not include pregnant or breastfeeding women. Ask your healthcare provider before using benfotiamine.
Who should talk to a clinician first: anyone with a chronic health condition, liver concerns, an upcoming surgery or regular prescription medication, and older adults taking several medicines.
Warning signs: Stop and seek care for signs of an allergic reaction. Report persistent stomach upset, dizziness or unusual tiredness to your clinician.

Frequently Asked Questions

Does benfotiamine have side effects?

It can, but trials have found few. In placebo-controlled studies of 300 to 600 mg a day for up to 24 months, side effects were no more common than with placebo and none were serious. A phase I study recorded small lab changes, such as a rise in the liver enzyme ALT.

Is benfotiamine safe for long-term use?

Trials of up to 24 months found it well tolerated. Its safety over many years, or at doses above those studied, is not known. If you take it long term, review it with your healthcare provider from time to time.

What medications should not be taken with benfotiamine?

No well-studied interactions have been confirmed. NIH notes that furosemide and fluorouracil can lower thiamin levels, so people taking them should discuss thiamin with their doctor. Tell your pharmacist about benfotiamine before starting any new medicine.

Can benfotiamine be taken during pregnancy?

There is not enough research to establish its safety during pregnancy or breastfeeding. Pregnant and nursing women should ask a healthcare professional before using it.

What happens if you take too much benfotiamine?

Single doses up to 1,200 mg were tolerated in a short phase I study, but large amounts have not been studied long term. More is not known to be better. Stay within the amount on the label or the amount your clinician suggests.

How is benfotiamine different from regular thiamine?

Benfotiamine is a synthetic, fat-soluble derivative that the body converts to thiamin. It raises blood thiamin levels more than the same amount of water-soluble thiamin mononitrate or hydrochloride.

Sources

Disclosure: OPA Nutrition sells supplements, including products mentioned in this article. This content is for education only and is not medical advice. Statements about dietary supplements have not been evaluated by the Food and Drug Administration. These products are not intended to diagnose, treat, cure, or prevent any disease. Talk to your healthcare provider before using any supplement, especially if you take medication or have a medical condition.

Written and reviewed by Michael Gonzales, Founder, OPA Nutrition · Last reviewed: October 17, 2026 · How OPA researches and writes

Benfotiamine Side Effects: What Studies Report and What to Know

Key Takeaways