Vitamin B1 (Thiamine): Benefits, Dosage, Side Effects & Evidence

Q: What is Vitamin B1 (Thiamine) used for?

Vitamin B1 (Thiamine) is most often taken for Corrects and prevents deficiency syndromes — beriberi (cardiac and neuropathic) and, with prompt IV dosing, Wernicke-Korsakoff syndrome in alcohol-use disorder, Repletes high-risk groups: heavy alcohol use, chronic loop-diuretic (furosemide) therapy, bariatric surgery, hyperemesis, refeeding syndrome, and critical illness, Essential cofactor (as thiamine pyrophosphate) for pyruvate and alpha-ketoglutarate dehydrogenase and transketolase, enabling carbohydrate energy metabolism, Benfotiamine (lipid-soluble form) gives modest, dose-dependent symptom relief in diabetic polyneuropathy (BENDIP) — though not a benefit of standard thiamine in non-deficient people. The energy-metabolism spark plug — vital when deficient, inert when you're not

Q: Does Vitamin B1 (Thiamine) work — what does the evidence say?

Strong evidence. Multiple high-quality RCTs / meta-analyses with consistent effects. Thiamine (vitamin B1) is an essential water-soluble vitamin whose active form, thiamine pyrophosphate, is a coenzyme for pyruvate dehydrogenase, alpha-ketoglutarate dehydrogenase, and transketolase — reactions central to carbohydrate metabolism and ATP production. Deficiency causes beriberi (wet/cardiac and dry/neuropathic forms) and, classically in alcohol-use disorder, Wernicke-Korsakoff syndrome; high-risk groups include heavy drinkers, those on chronic loop diuretics, bariatric-surgery and critically ill patients. In non-deficient people, supplementation trials are largely null: a 2024 meta-analysis of 7 RCTs found no improvement in LVEF in chronic heart failure, and high-dose thiamine did not relieve chronic fatigue versus placebo in a crossover RCT. The lipid-soluble derivative benfotiamine shows modest, dose-dependent symptom benefit in diabetic polyneuropathy but evidence remains limited.

Q: How many studies support Vitamin B1 (Thiamine)?

NutriDex cites 12 sources for Vitamin B1 (Thiamine), graded "Strong".

Peh, Daryl

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Vitamin B1 (Thiamine)

The energy-metabolism spark plug — vital when deficient, inert when you're not

Strong evidence 🍊Vitamin

✚ Updated 16 Aug 2026 · How we grade evidence

Evidence tier

Strong

Research weight

Citations

12 verified / 12

Classification

Vitamin

❝

What the evidence says. Multiple high-quality RCTs / meta-analyses with consistent effects.

What is Vitamin B1 (Thiamine)?

Vitamin B1 (Thiamine) is a vitamin used for corrects and prevents deficiency syndromes — beriberi (cardiac and neuropathic) and, with prompt iv dosing, wernicke-korsakoff syndrome in alcohol-use disorder. NutriDex grades the human evidence as Strong. Thiamine (vitamin B1) is an essential water-soluble vitamin whose active form, thiamine pyrophosphate, is a coenzyme for pyruvate dehydrogenase, alpha-ketoglutarate dehydrogenase, and transketolase — reactions central to carbohydrate metabolism and ATP production. Deficiency causes beriberi (wet/cardiac and dry/neuropathic forms) and, classically in alcohol-use disorder, Wernicke-Korsakoff syndrome; high-risk groups include heavy drinkers, those on chronic loop diuretics, bariatric-surgery and critically ill patients. In non-deficient people, supplementation trials are largely null: a 2024 meta-analysis of 7 RCTs found no improvement in LVEF in chronic heart failure, and high-dose thiamine did not relieve chronic fatigue versus placebo in a crossover RCT. The lipid-soluble derivative benfotiamine shows modest, dose-dependent symptom benefit in diabetic polyneuropathy but evidence remains limited.

Purported Benefits

✓Corrects and prevents deficiency syndromes — beriberi (cardiac and neuropathic) and, with prompt IV dosing, Wernicke-Korsakoff syndrome in alcohol-use disorder

✓Repletes high-risk groups: heavy alcohol use, chronic loop-diuretic (furosemide) therapy, bariatric surgery, hyperemesis, refeeding syndrome, and critical illness

✓Essential cofactor (as thiamine pyrophosphate) for pyruvate and alpha-ketoglutarate dehydrogenase and transketolase, enabling carbohydrate energy metabolism

✓Benfotiamine (lipid-soluble form) gives modest, dose-dependent symptom relief in diabetic polyneuropathy (BENDIP) — though not a benefit of standard thiamine in non-deficient people

✓No proven benefit in heart failure beyond correcting documented deficiency: a 2024 meta-analysis of 7 RCTs found no LVEF improvement

✓No benefit for chronic fatigue or energy in replete individuals despite popular 'energy vitamin' marketing

Evidence by outcome

The same supplement can be well-proven for one use and unproven for another — here is the human evidence graded outcome by outcome.

Outcome	Evidence	Effect	Studies
Treatment of deficiency syndromes (beriberi, Wernicke-Korsakoff)IV/oral thiamine reverses beriberi and Wernicke encephalopathy; evidence is observational/case-series but consistent, optimal dose still undefined.	Strong	↑ benefit · large	3
Repletion in high-risk groups (alcohol, diuretics, bariatric, refeeding)Systematic reviews confirm deficiency is common in these groups and repletion is indicated; benefit is preventing deficiency, not enhancement.	Moderate	↑ benefit · moderate	2
Diabetic polyneuropathy (benfotiamine)Single RCT (BENDIP, n=165) showed dose-dependent symptom relief, but per-protocol p=0.033 / ITT p=0.055; applies to benfotiamine, not standard thiamine.	Preliminary	↑ benefit · small	1
Heart failure (LVEF in non-deficient)2024 meta-analysis of 7 RCTs (n=274) found no LVEF improvement (WMD 1.65%, p=0.24) beyond correcting documented deficiency.	Moderate	— no effect · negligible	1
Chronic fatigue / energy in replete individualsCrossover RCT in primary biliary cholangitis found high-dose thiamine no better than placebo for fatigue; contradicts 'energy vitamin' marketing.	Preliminary	— no effect · negligible	1
Septic shock mortalityMeta-analysis of RCTs with trial sequential analysis found thiamine alone did not reduce mortality (RR 0.87, 95% CI 0.65-1.16).	Moderate	— no effect · negligible	1

Dosing & Compounds

Typical Dose

Adult RDA: 1.2 mg/day (men), 1.1 mg/day (women); 1.4 mg/day in pregnancy and lactation. Typical supplements supply 1.5-100 mg. Therapeutic doses are far higher — e.g., 100 mg+ IV thiamine for Wernicke encephalopathy, or benfotiamine 300-600 mg/day in neuropathy trials. No Tolerable Upper Intake Level (UL) has been established; excess is readily excreted in urine.

Active Compounds

Thiamine hydrochloride (mononitrate) — standard oral supplement and food-fortification formBenfotiamine — lipid-soluble S-acyl derivative with higher bioavailability, studied in diabetic neuropathyThiamine HCl injection — IV/IM for Wernicke encephalopathy and severe deficiencyDietary sources: whole and enriched/fortified grains, pork, legumes, nuts, seeds, yeast, and fortified cereals

Safety & Cautions

Thiamine has very low toxicity and no established UL — excess oral intake is excreted renally, and adverse effects from food or supplements have not been documented. Rare anaphylactoid reactions have been reported with rapid high-dose IV administration. In suspected Wernicke encephalopathy, thiamine must be given before or with glucose, since a glucose load in a thiamine-depleted patient can precipitate or worsen encephalopathy. Chronic loop diuretics (furosemide) and heavy alcohol use increase urinary loss/impair absorption and raise deficiency risk; no major drug interactions limit supplementation itself. Educational only — always check with your doctor or pharmacist before combining Vitamin B1 (Thiamine) with any medicine.