Thiamine tablets: what vitamin B1 does

What are thiamine tablets used for?

Thiamine tablets top up vitamin B1, and as a licensed medicine they treat or prevent vitamin B1 deficiency. Thiamine, also spelt thiamin, helps the body break down and release energy from food and helps keep the nervous system healthy, according to the NHS.

The body keeps only small stores. The UK's Expert Group on Vitamins and Minerals (EVM) notes that a marginal deficiency can develop quite rapidly, so a regular intake matters. Most adults get enough from food, and the NHS says you should be able to get all the thiamin you need from a varied and balanced diet. Tablets fall into two groups, food supplements you can buy yourself and licensed medicines a doctor or pharmacist supplies, and that difference is covered further down.

How much thiamine do you need?

The NHS gives the amount for adults aged 19 to 64 as 1 mg a day for men and 0.8 mg a day for women.

Which foods contain thiamine?

The NHS lists peas, some fresh fruit such as bananas and oranges, nuts, wholegrain bread, some fortified breakfast cereals and liver (avoid liver if you are pregnant). UK food composition data show where the larger amounts sit.

Pork stands out among meats: grilled lean loin gives several times the thiamin of beef or chicken in the same dataset. White bread scores about the same as wholemeal because UK flour is fortified with thiamin by law, a requirement the EVM described in 2003. Portions differ a lot, so use the table to compare foods rather than to count milligrams.

What does thiamine do? The authorised statements

Four statements are authorised for thiamine on the Great Britain nutrition and health claims register.

These describe the part thiamine plays in normal body function. They don't say a supplement boosts energy or treats a condition. If you're always tired, our guide to the causes and blood tests is a better place to start.

What are the symptoms of vitamin B1 deficiency?

Early symptoms are vague, and deficiency is uncommon in people who eat a varied diet. A 2021 clinical review lists severe fatigue, weakness, nausea, vomiting, loss of appetite and abdominal discomfort, and names nerve problems in the hands and feet as an early sign in adults. The classic severe forms are beriberi, which can affect the heart and the nerves, and Wernicke's encephalopathy, a brain condition with confusion, abnormal eye movements and unsteady walking that can progress to Korsakoff syndrome, with memory and thinking problems. The review says no single test is diagnostic, so doctors rely on suspicion and may treat early, and improvement can come within hours or days. Risk is higher with heavy drinking, weight-loss surgery, severe vomiting such as hyperemesis in pregnancy, eating disorders and long-term malnutrition. These symptoms overlap with many other conditions, so see a GP rather than treating yourself, and get urgent medical help for confusion with eye or balance problems.

How much thiamine is too much?

The NHS says taking 100 mg or less a day of thiamine supplements is unlikely to cause harm, and that there isn't enough evidence to know the effects of higher doses. The EVM's 2003 review couldn't set a safe upper level for thiamine. It used 100 mg a day as a guidance level, based on a trial in which 556 girls and young women took 100 mg a day for up to 90 days with no adverse effects reported. It also noted that water-soluble forms of thiamine are poorly absorbed at high doses, which limits their toxicity.

Daily amounts add up across products. Many multivitamins and B complexes contain thiamine, so check every label before adding a separate tablet. Our B complex guide shows the common amounts.

What are the side effects of thiamine?

The NHS lists nausea, diarrhoea and stomach ache as the common side effects. For nausea it suggests taking thiamine with, or just after, a meal or snack, and for diarrhoea drinking plenty of fluids. Speak to a pharmacist or doctor if a side effect doesn't go away. Rarely, thiamine causes a serious allergic reaction: swelling of the lips, mouth, throat or tongue, fast or difficult breathing, a rash, or sudden confusion or drowsiness. The NHS says to call 999 straight away.

The EVM describes the oral toxicity of thiamine as very low. The product information for licensed thiamine tablets lists mild nausea, vomiting, diarrhoea and abdominal pain, and allergic reactions that are mostly reported after injections. The EVM found only a handful of case reports of reactions to oral thiamine, one of them a fatal anaphylactic reaction after a 100 mg dose, so anyone who has reacted to a B vitamin before should ask a pharmacist. The fluorouracil chemotherapy medicine can cancel out thiamine, so people on it should check with their treatment team.

Can you buy thiamine over the counter?

Yes, as a food supplement. The NHS says you can buy thiamine supplements from pharmacies or shops. Licensed thiamine tablets are a different thing: they are medicines, and thiamine can also be prescribed.

A licensed thiamine hydrochloride 100 mg tablet listed on the electronic medicines compendium is indicated for the treatment of thiamine deficiency and is classed as a prescription-only medicine. Shops also sell 100 mg tablets as food supplements. The molecule is the same, but the legal status and what the label may say differ.

The MHRA's guidance is that food supplements shouldn't refer to nutritional deficiencies or call consumers patients, and that a supplement making medicinal claims can be treated as a medicine. So a supplement label that promises to fix a deficiency is a warning sign.

Why do doctors give thiamine to people who drink heavily?

Heavy drinking can leave the body short of thiamine, and a severe shortage can damage the brain, so doctors offer thiamine to prevent that. Alcohol impairs how the body takes up and uses thiamine, and people who drink heavily often eat poorly as well. The EVM describes the main danger as Wernicke's encephalopathy, a condition marked by confusion, unsteadiness and, in the worst cases, coma, sometimes followed by Korsakoff psychosis.

NICE advises offering thiamine to people at high risk of Wernicke's encephalopathy. For heavy or dependent drinkers that means those who are malnourished or at risk of it, who have severe liver disease, who are in withdrawal, or who are about to start a medically supervised withdrawal. NICE says suspected Wernicke's encephalopathy should be treated with thiamine by injection and that doctors should keep a high level of suspicion. The doses used are a medical decision and not something to copy from a shop supplement.

Where Natura fits

Daily Thiamine (25 mg) is in the Natura cabinet (coming soon), as one of the two ingredients in the Memory Pack, a food supplement that isn't an Alzheimer's medicine. At 25 mg it is well below the NHS 100 mg level, but Daily Multi also contains thiamin (50 mg), so taking the two together gives 75 mg a day.