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Vitamin B1 (Thiamine)

Thiamine is life-saving in deficiency states — Wernicke's encephalopathy, beriberi, refeeding syndrome — and deficiency is common in alcohol use disorder, after bariatric surgery, and in diuretic-treated heart failure. Outside those settings, supplementing does not improve energy, mood, or performance.

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What the science says

  • Treats and prevents Wernicke's encephalopathy Strong evidence

    European Federation of Neurological Societies guidance recommends parenteral thiamine 200 mg three times daily for suspected Wernicke's, given before any glucose load, because untreated cases progress to irreversible Korsakoff syndrome or death. The evidence base is largely clinical and observational, but the effect size in acute deficiency is unambiguous and no ethical placebo trial exists.

  • Modestly improves ejection fraction in heart failure Moderate evidence

    A meta-analysis of randomised trials in heart failure patients found thiamine supplementation improved left ventricular ejection fraction by roughly 3-5 percentage points versus placebo, plausibly because loop diuretics increase urinary thiamine loss. Trials were small and short, and no mortality benefit has been shown, so this stays moderate at best.

  • Boosts energy or reduces fatigue in people with adequate intake Limited evidence

    Thiamine is a coenzyme in carbohydrate metabolism, which drives a lot of 'energy vitamin' marketing, but no controlled trial shows added thiamine improves subjective energy, cognition, or exercise capacity in replete adults. Correcting deficiency helps; topping up a normal level does not.

  • Improves diabetic complications (benfotiamine) Limited evidence

    Benfotiamine, a lipid-soluble thiamine derivative, showed early promise for diabetic neuropathy and microvascular endpoints at 300-600 mg/day, but subsequent trials have been inconsistent and generally failed to show meaningful change in HbA1c or nerve conduction. Treat as unproven.

Dosage & safety

Studied doseRDA is 1.2 mg/day for men and 1.1 mg/day for women. Typical supplement doses are 50-100 mg/day. Heart failure trials used 100-300 mg/day orally for 4-12 weeks. Wernicke's encephalopathy protocols use 200-500 mg IV three times daily for 2-3 days, then oral maintenance. Benfotiamine studies used 300-600 mg/day.
SafetyOral thiamine is extremely well tolerated with no established upper limit — excess is excreted in urine. Rare anaphylaxis has been reported with rapid intravenous administration, so IV dosing belongs in a clinical setting. Give thiamine before or alongside IV glucose in at-risk patients (alcohol use disorder, malnutrition, hyperemesis), since a glucose load in a thiamine-deficient person can precipitate Wernicke's. Loop diuretics, chronic alcohol use, bariatric surgery and prolonged vomiting all increase requirements. No meaningful drug interactions at supplemental doses.

How to take it

TimingTiming does not matter.
With food?With or without food; it is water-soluble and absorbed either way.
Worth knowingThiamine is one of the few B vitamins genuinely worth supplementing without testing if you drink heavily, have had bariatric surgery, or take a loop diuretic long-term.

Interactions

With medications

  • loop diuretics (furosemide)

    Long-term use increases urinary thiamine loss and can produce deficiency, particularly in heart failure. a maintenance supplement is reasonable on prolonged loop diuretic therapy — discuss with your doctor

  • intravenous glucose in at-risk patients

    A glucose load given to a thiamine-deficient person can precipitate Wernicke's encephalopathy. thiamine must be given before or alongside IV glucose — a hospital protocol, not something to manage yourself

Well-established interactions only — this is not a complete list. Always tell your doctor and pharmacist what you take.

Frequently asked questions

Who should take a vitamin B1 (thiamine) supplement?

Thiamine is worth taking for people at real risk of deficiency: heavy drinkers, people who have had bariatric surgery, and those on long-term loop diuretics. For these groups vitamin B1 is one of the few B vitamins worth supplementing without testing first.

Does vitamin B1 give you more energy?

Not if your intake is already adequate — the evidence is limited. No controlled trial shows extra thiamine (vitamin B1) improves energy, cognition or exercise capacity in people who are not deficient.

Does thiamine help heart failure?

The evidence is moderate and on a surrogate measure: a meta-analysis found thiamine raised ejection fraction by roughly 3–5 percentage points, possibly because loop diuretics drain it. The trials were small and short and showed no mortality benefit.

How much vitamin B1 per day, and is it safe?

The RDA for thiamine is 1.2 mg/day for men and 1.1 mg/day for women; typical supplements provide 50–100 mg/day. Oral vitamin B1 is extremely well tolerated with no established upper limit, since the excess is excreted in urine.

Scientific references

Where to buy

NOW Foods, B-1, 100 mg, 100 Tablets

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