Quick answer
Ask your pharmacist or doctor before combining. Furosemide, a diuretic, has been linked to lower thiamin levels because more thiamin leaves the body in urine. In a study of 6 volunteers, small doses doubled the rate of thiamin loss for several hours. NIH advises regular users to discuss their thiamin status with their providers.1,2
Key facts
- NIH's consumer fact sheet names furosemide among the medicines that can lower thiamin levels in the body.3
- The extra loss follows urine flow: in volunteers, a saline drip that raised urine output had the same effect as furosemide.2
- In 30 outpatients with heart failure on standard loop diuretic doses, under 12% had low thiamin, and the dose did not track thiamin levels.4
- The furosemide label we read does not mention thiamin; it warns about losses of water and electrolytes.5
Your doctor may want to check on you
Furosemide, a loop diuretic, has been linked in research to lower thiamin levels, possibly low enough to be deficient, because more thiamin is lost in urine. NIH says people who take it regularly should discuss their thiamin status with their health care providers.1,2,3
Reported in studies
What is documented
NIH's fact sheet for health professionals says research has linked furosemide, a loop diuretic, to falling thiamin levels, possibly to deficient levels, through thiamin lost in urine. It adds that clinical studies have not yet shown whether thiamin supplements keep levels up in people taking loop diuretics.1
This is reported in studies. It is not on the furosemide label: the label we read warns that the medicine can cause heavy losses of water and of electrolytes such as potassium and magnesium, and asks for regular blood tests, but it does not mention thiamin.5
Why it happens
Thiamin dissolves in water, and the kidneys quickly filter out the thiamin in blood that is not bound to proteins. In 6 healthy volunteers, single small doses of furosemide given into a vein raised urine flow and roughly doubled the rate at which thiamin left in urine; the rate returned to normal over the next 18 hours. A saline drip that raised urine flow had the same effect, so the loss tracked the amount of urine rather than the medicine itself.2,6
A 2016 review of the research concluded that low thiamin in people on long-term furosemide comes mainly from higher urine volume and flow, and that the medicine may also reduce thiamin uptake into cells.7
Who is most likely to be affected
Most of the research is in people with heart failure, who often take furosemide for years. NIH reports that studies have found poor thiamin status in 21% to 98% of these patients, explained by older age, other illnesses, low intake, diuretic treatment and frequent hospital stays.1
Studies disagree on the size of the problem. In 30 stable outpatients on standard loop diuretic doses, the estimated share with low thiamin was under 12%, and the diuretic dose did not track thiamin levels.4
The volunteer study points to a long stretch of high urine output, together with a diet low in thiamin, as the situation to watch.2
Other diuretics
NIH names furosemide, a loop diuretic. In rats, the extra loss depended on how much urine was produced rather than on the type of diuretic, but the human research has focused on furosemide. The sources on this page do not address thiazide or potassium-sparing diuretics directly. That is not a finding either way, so ask your pharmacist about your own medicine.1,2
What to ask your pharmacist or doctor
- Could my thiamin be low, and is it worth checking?
- Does my diet give me enough thiamin, or do I need a supplement?
- Is a multivitamin or B vitamin suitable while I take furosemide?
- Do any of my other medicines affect thiamin or other vitamins?
MedlinePlus advises telling your doctor and pharmacist about all the vitamins, nutritional supplements and herbal products you take or plan to take while on furosemide. Bring the labels of everything you use.8
Questions people ask
Should I take thiamine if I take furosemide?
That is a question for your doctor. NIH says studies have not yet shown whether thiamin supplements keep levels up in people taking loop diuretics. Your doctor can weigh your diet, your medicines and any signs of low thiamin.1
Does furosemide affect other vitamins and minerals?
The furosemide label warns about losses of water and electrolytes, including potassium and magnesium, and asks for regular blood tests. Ask your doctor or pharmacist which of these apply to you.5
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Sources
- National Institutes of Health, Office of Dietary Supplements. Thiamin: Fact Sheet for Health Professionals. Updated February 9, 2023. Opened 2026-10-07.
- Rieck J, Halkin H, Almog S, et al. Urinary loss of thiamine is increased by low doses of furosemide in healthy volunteers. J Lab Clin Med. 1999;134(3):238-243. Opened 2026-10-07.
- Office of Dietary Supplements, National Institutes of Health. Thiamin: Fact Sheet for Consumers. Updated March 22, 2021. Opened 2026-10-07.
- Teigen LM, Twernbold DD, Miller WL. Prevalence of thiamine deficiency in a stable heart failure outpatient cohort on standard loop diuretic therapy. Clin Nutr. 2016;35(6):1323-1327. Opened 2026-10-07.
- Furosemide tablets: prescribing information (RemedyRepack). DailyMed, U.S. National Library of Medicine. Published September 17, 2026. Opened 2026-10-07.
- EFSA Panel on Dietetic Products, Nutrition and Allergies (NDA). Dietary reference values for thiamin. EFSA J. 2016;14(12):e04653. Opened 2026-10-07.
- Katta N, Balla S, Alpert MA. Does long-term furosemide therapy cause thiamine deficiency in patients with heart failure? A focused review. Am J Med. 2016;129(7):753.e7-753.e11. Opened 2026-10-07.
- MedlinePlus, U.S. National Library of Medicine. Furosemide. Last revised April 15, 2025. Opened 2026-10-07.
This page is information, not medical advice. Talk to your doctor or pharmacist about your own situation.