Thiamine (vitamin B1): uses, dosage, side effects and refills

Thiamine, also spelled thiamin and called vitamin B1, is one of the water-soluble B vitamins. The body turns it into thiamine diphosphate, a helper molecule that five enzymes need to burn glucose and process amino acids and fats (NIH Office of Dietary Supplements, ODS).

The adult body holds only about 25 to 30 mg, mostly in the liver, and the vitamin has a short half-life, so you need a steady supply from food (ODS). The injection label notes that stores can run out after roughly three weeks with no thiamine at all, and that the need rises when a diet is heavy in carbohydrate.

Most Americans get enough: only 6% have a usual intake below the average requirement, largely because US bread, cereal and infant formula are fortified (ODS). Pork, whole grains, fish and black beans are good natural sources.

Over the counter, thiamine is sold as a dietary supplement in tablets of thiamine hydrochloride or thiamine mononitrate, usually 100 mg, many times the adult daily need. Hospitals use an Rx-only injection, 100 mg/mL, when levels must be restored quickly. Benfotiamine is a related synthetic form found in some supplements.

FDA-labeled uses (thiamine hydrochloride injection, Rx only)

• Treatment of thiamine deficiency or beriberi, both the "dry" nerve form and the "wet" heart form.
• Rapid replacement when it is urgent: Wernicke's encephalopathy, infantile beriberi with collapse, heart disease from thiamine deficiency, and neuritis of pregnancy with severe vomiting.
• Before or with IV dextrose in people with marginal thiamine status, to avoid triggering heart failure.
• Established deficiency in people who cannot take thiamine by mouth because of severe anorexia, nausea, vomiting or malabsorption. (All from the thiamine hydrochloride injection label, Indications.)

Oral thiamine (dietary supplement, not FDA-approved for a disease)

• Mild deficiency: the World Health Organization regimen cited by ODS is 10 mg a day for a week, then 3 to 5 mg a day for at least 6 weeks.
• Prevention in high-risk groups: up to 80% of people with chronic alcohol use disorder develop deficiency; 20% to 30% of older adults show lab signs of it; supplements with thiamine are almost always recommended after bariatric surgery (ODS).
• Alcohol use without signs of Wernicke's: the Royal College of Physicians supports oral thiamine 100 mg three times a day (ODS).

Studied, not proven (off-label)

• Heart failure: 21% to 98% of patients have poor thiamine status in studies, and two small trials showed better ejection fraction, with unclear clinical meaning (ODS).
• Diabetes: small studies of 150 to 300 mg a day lowered glucose; benfotiamine for neuropathy gave mixed results (ODS).
• Alzheimer's disease: trials were too small to draw conclusions (ODS).

Inside cells, thiamine joins with ATP to form thiamine pyrophosphate, also called cocarboxylase (thiamine injection label). That coenzyme lets the body pull energy out of carbohydrate: it drives the step that converts pyruvate into fuel for the mitochondria, and related enzymes, including transketolase in red cells, handle alpha-keto acids and other sugars (injection label; ODS).

Without enough thiamine, pyruvate and lactate build up and the cells that burn the most glucose, nerves and heart muscle, run short of energy. That explains the two faces of deficiency: nerve damage and confusion (dry beriberi, Wernicke's) and heart failure with swelling (wet beriberi). It is also why giving a sugar-containing IV drip to someone with low thiamine can tip them into heart failure or Wernicke's, and why the label asks for thiamine with dextrose.

Absorption from the gut uses an active carrier at food-level doses and passive diffusion at high doses (ODS). The carrier saturates quickly, so the Food and Nutrition Board notes absorption drops sharply above about 5 mg, and extra is passed in urine.

Oral thiamine is very safe, and the injection label lists only one contraindication: a history of sensitivity to thiamine or any ingredient in the product. Tell your clinician or pharmacist before using it if any of these apply:

• You have had a reaction to a thiamine shot. Serious and even fatal allergic reactions have followed IV or IM thiamine, mostly after repeated doses; a test dose is used if allergy is suspected (thiamine injection label, Warnings).
• You drink heavily or have recently stopped. You may need more than an over-the-counter tablet, and confusion, unsteady walking or abnormal eye movements need emergency care, not home treatment.
• You have had bariatric surgery, persistent vomiting, or a condition that blocks absorption (ODS).
• You take a loop diuretic such as furosemide; it increases thiamine loss in urine (ODS).
• You are receiving fluorouracil chemotherapy; cases of beriberi and Wernicke's have been reported (ODS).
• You have kidney disease and need injections: the injection contains aluminum, which can build up when kidneys are impaired (label, Warnings).
• You are pregnant with severe vomiting (hyperemesis); the label covers injected thiamine for this.

Ask: "Do my symptoms point to low thiamine, and should I also take a B-complex or multivitamin?" The label notes that pure B1 deficiency is rare and several vitamins are often low together.

Take thiamine exactly as the package or your clinician directs.

• Tablets: a typical supplement label says to take 1 tablet daily with water and a meal (Nature Made Vitamin B1 100 mg). Food is not required for absorption, but taking it with a meal makes it easier to remember.
• Swallow tablets with a full glass of water. Ask a pharmacist before splitting or crushing anything labeled extended-release.
• Keep taking it as long as the plan says. The injection label pairs short-term shots with an oral multivitamin for a month to refill body stores, and stresses correcting the diet so deficiency does not return.
• Injections are given by a nurse or doctor, into a muscle or slowly into a vein. Expect some tenderness or firmness at a muscle site. You will usually be watched for at least 30 minutes after an injection if there is any concern about allergy (label, Warnings).
• If you are being treated for alcohol-related deficiency, oral doses may not raise blood levels enough when absorption is impaired; experts favor injections for anyone with confusion, unsteady gait or eye-movement problems (ODS, citing European and UK guidelines).
• Do not substitute benfotiamine for prescribed thiamine without asking; it is a different compound.

Your clinician sets the dose and the route. Over-the-counter 100 mg tablets supply far more than the 1.1 to 1.2 mg adults need each day, and the injection label notes that doses above 30 mg three times a day are not used effectively.

• No upper limit: high oral intakes have produced no reported harm, so the Food and Nutrition Board never fixed a ceiling for thiamine (ODS).
• Kidney disease: oral thiamine needs no adjustment, but repeated injections expose the body to aluminum, a risk when kidney function is impaired (label).
• Older adults: up to 20% to 30% have lab signs of low thiamine, linked to diet, illness and multiple medicines (ODS); ask whether a supplement makes sense.
• After bariatric surgery: thiamine is part of the standard vitamin plan; your surgical team sets the amount.

Ask your prescriber: "How long should I take thiamine, and will you check whether it is working?"

If you miss a daily thiamine tablet, take it when you remember that day. If it is already the next day, skip the missed one and carry on with your usual schedule. Do not take two doses to catch up. A single missed tablet is unlikely to matter because a 100 mg tablet supplies many times the daily need.

Oral thiamine overdose is very unlikely to cause harm: the body stops absorbing much above about 5 mg and excretes the excess in urine (ODS), and single injected doses of 100 to 500 mg have been given without toxic effects (thiamine injection label). If someone has swallowed a large amount of a multi-ingredient product, other ingredients may be the concern. Poison Help (1-800-222-1222) can advise on any large swallow; anyone who collapses, struggles to breathe or swells up after a shot needs 911.

• Heavy alcohol use. Alcohol cuts thiamine absorption from the gut, reduces liver stores and blocks its activation, which is why up to 80% of people with chronic alcohol use disorder become deficient (ODS).
• Treating confusion at home. New confusion, unsteady walking or double vision in someone at risk needs emergency care and injected thiamine.
• Throwing out cooking water from grains and vegetables. Thiamine dissolves in water and is lost (ODS).
• Relying on unenriched white rice. It has one-tenth the thiamine of brown rice (ODS).
• Long fasts or very restrictive diets without a vitamin plan. Body stores can run out after about three weeks with no thiamine intake (thiamine injection label).

Pregnancy: the need for thiamine rises to 1.4 mg a day (ODS), and most prenatal vitamins cover it. The injection label says studies in pregnant women have not shown an increased risk of fetal abnormalities, so harm "appears remote", though it advises use only if clearly needed. Severe pregnancy vomiting can cause deficiency; the label lists injected thiamine for neuritis of pregnancy.

Breastfeeding: the recommended intake is also 1.4 mg a day (ODS). The injection label says it is not known whether injected thiamine passes into milk and advises caution. Infants can develop beriberi when a breastfeeding mother is deficient (MedlinePlus, Beriberi), so adequate maternal intake matters.

Keep thiamine tablets at room temperature, tightly closed, away from moisture and out of children's reach. The injection is stored at 20°C to 25°C (68°F to 77°F) and protected from light, and is used only if the solution is clear (thiamine injection label, How Supplied).

Oral thiamine is inexpensive and does not need a prescription. Nature Made's list price for Vitamin B1 100 mg, 100 tablets, is $7.39, about 7 cents a tablet (Nature Made product page, checked 6 Oct 2026). Store brands at pharmacies and supermarkets are often similar or lower; compare the per-tablet price.

Mark Cuban Cost Plus Drugs does not list oral thiamine (Cost Plus Drugs price list, checked 6 Oct 2026). Thiamine injection is given in clinics and hospitals and billed as part of that care rather than bought at a counter.

Insurance plans usually do not cover OTC vitamins, though a health savings or flexible spending account may. If thiamine is part of a bariatric or alcohol-treatment plan, ask your care team whether a covered combination product is available.

Related vitamins and medicines, each with its own page:

• Cyanocobalamin: vitamin B12, often low alongside B1 after bariatric surgery.
• Folic acid: another B vitamin commonly depleted with heavy drinking.
• Furosemide: a loop diuretic linked to thiamine loss.
• Torsemide: a loop diuretic used for heart failure.
• Fluorouracil: a cancer drug reported to cause thiamine deficiency.
• Ascorbic acid: vitamin C, another water-soluble vitamin sold OTC.

No prescription is needed for oral thiamine. Pharmacies and grocery stores stock 100 mg tablets on the vitamin shelf, so Temi plays no part in buying them. Thiamine injections are given in clinics and hospitals, and suspected Wernicke's encephalopathy needs emergency care, not online care. What Temi can do is renew a non-controlled prescription that affects thiamine, such as furosemide: a physician reviews the request within 48 hours and the prescription goes to your chosen pharmacy. The refill review is $15, or free for Temi Care members (prescription refills, checked 6 Oct 2026).