Vitamin B1 (Thiamine) for Thiamine (B1) Deficiency

Thiamine deficiency is treated with thiamine, and nothing else substitutes for it. The question this page answers is how well that is proven. The case for replacing thiamine rests on biology, guidelines and clinical experience more than on large randomised trials. A Cochrane review found only two randomised trials of thiamine for Wernicke-Korsakoff syndrome in people who misuse alcohol, and concluded the evidence was insufficient to guide dose, frequency, route or duration (Cochrane review, 2013).

What the research does show is consistent. A double-blind trial in 90 Cambodian women found that thiamine-fortified fish sauce raised red-cell thiamine diphosphate (the active form) and breast-milk thiamine over 6 months, and the higher-concentration sauce raised it in their infants too (Cambodian fish-sauce trial, 2016). A systematic review of 20 case reports found symptoms partly or fully resolved in 18 (90%) after intravenous thiamine (2024 systematic review). That pattern supports replacement as effective, and the open questions are about how much, by which route, and for how long.

Thiamine is converted in the body to thiamine diphosphate (TDP), its active form, which helps cells turn food into energy (NIH Office of Dietary Supplements). The brain, nerves and heart use energy heavily, which is why deficiency shows up as nerve damage (dry beriberi), heart failure with fluid retention (wet beriberi) and the brain disorder Wernicke encephalopathy (NIH Office of Dietary Supplements).

Taking thiamine addresses the cause directly: a shortage. At normal intakes the gut absorbs it by active transport, and at very high doses by passive diffusion, and the body excretes the excess in urine (NIH Office of Dietary Supplements). That is why injected doses are used when the gut cannot be relied on, as with persistent vomiting or after bariatric surgery.

Knowing why thiamine is needed doesn't tell you how much a given person needs. The Cochrane review found no clear dose-response pattern in the one trial it could analyse: in 107 people given one of five intramuscular doses for 2 days, 200 mg a day beat 5 mg a day on one memory test (P = 0.04), but the other doses did not, and the study had methodological shortcomings (Cochrane review, 2013).

Damage that has already happened may not reverse. The NIH notes that in the chronic Korsakoff stage, injected thiamine does not lead to recovery in about one-quarter of patients (NIH Office of Dietary Supplements). Early treatment matters more than the exact dose.

No dose here comes from a trial that tested one regimen against another in deficiency; they come from guidelines and clinical practice. The WHO regimen summarised by the NIH is 10 mg of thiamine by mouth daily for a week, then 3-5 mg daily for at least 6 weeks, for mild deficiency (NIH Office of Dietary Supplements). For severe deficiency the NIH lists 50-100 mg by injection in adults, then 10 mg daily intramuscularly for about a week, then 3-5 mg daily by mouth for at least 6 weeks (NIH Office of Dietary Supplements).

For suspected Wernicke encephalopathy, European guidelines recommend 200 mg three times a day, before any carbohydrate, preferably intravenously (EFNS guideline, 2010). Giving glucose first can worsen the condition: intravenous glucose made Wernicke encephalopathy worse in 14.1% of 177 pregnancy cases reviewed (2019 hyperemesis review).

The best controlled evidence is in pregnancy and breastfeeding in a rice-eating community. In the fish-sauce trial, 90 pregnant women were randomised to control, 2 g/L or 8 g/L thiamine-fortified fish sauce for 6 months. Baseline-adjusted red-cell thiamine diphosphate was higher in both thiamine groups (282 nM and 254 nM) than in controls (193 nM; P < .05), and infants in the high group had higher levels (257 nM) than low (212 nM) or control (187 nM) infants (Cambodian fish-sauce trial, 2016).

A larger trial gave 335 breastfeeding mothers 0, 1.2, 2.4 or 10 mg a day for 22 weeks. At 10 mg a day infants showed better language development by 6 months, but generally not motor development (Cambodian maternal-dose trial, 2021). A secondary analysis found a small dose-response relationship with infants' attention to speech (adjusted R-squared 0.022, p = .011) (2025 secondary analysis). These show benefit from higher intakes in an at-risk population, not that every low reading needs a high dose.

The 2010 European guideline recommends thiamine for any suspected or manifest Wernicke encephalopathy, rates its overall safety as very good, and says it should be suspected whenever a condition could lead to deficiency, not only in people who drink alcohol (EFNS guideline, 2010). A 2021 review found 81 adults with confirmed deficiency not linked to alcohol in high-income countries, and most improved partly or fully with intravenous thiamine (2021 literature review).

The weakness is design. The 2024 systematic review found 27 studies, mostly case reports, with one randomised trial (2024 systematic review). In pregnancy, 63.6% of women in 177 reported Wernicke cases received a subtherapeutic thiamine dose, and 65.4% had lasting cognitive problems, so under-treatment is a documented problem (2019 hyperemesis review).

A meta-analysis of 11 studies in 1,494 patients found 27% had thiamine deficiency after bariatric surgery (2022 meta-analysis). A later analysis of 41 studies found it was 7% before surgery, then 19% at 3 months, 9% at 6 months and 6% at 1 year, so the risk is concentrated in the early months (2024 meta-analysis). Among 778 sick infants in Laos with no clinical beriberi, 13.4% had low basal transketolase activity, a lab sign of deficiency (Laos infant cohort, 2011).

Thiamine has a very good safety record. The European guideline rates overall safety as very good (EFNS guideline, 2010), and the US Food and Nutrition Board set no upper limit because adverse effects from 50 mg a day or more of food or supplement thiamine were not reported; the body excretes the excess in urine (NIH Office of Dietary Supplements).

The real risk is delay, not toxicity: taking tablets for neurological symptoms in place of an assessment can let Wernicke encephalopathy progress (EFNS guideline, 2010).

The NIH says thiamine is not known to interact with any medications, but some drugs lower thiamine levels (NIH Office of Dietary Supplements). Furosemide, a loop diuretic, is linked to urinary thiamine loss, though whether supplements prevent deficiency in people taking loop diuretics still needs trials (NIH Office of Dietary Supplements). Cases of beriberi or Wernicke encephalopathy have followed fluorouracil chemotherapy (NIH Office of Dietary Supplements).