Dec232025
A woman holds her abdomen in pain from a stomach ache, a possible side effect of vitamin B1.

Does vitamin B1 have side effects?

Starting any supplement often brings a mix of emotions along with it: the hope for benefits, yes,  but also concerns about whether there might be unwanted side effects. When you’re considering taking vitamin B1 (thiamine), it’s completely understandable to ask “What could go wrong?” This article outlines the possible side effects of vitamin B1, how likely they are and whether a daily intake is acceptable for most people.

What are the side effects of vitamin B1?

Vitamin B1 is extremely well tolerated in the majority of people. Being a water‑soluble vitamin, the accumulation risk is generally low as the human body tends not to store large amounts and instead excretes excess.¹ Thiamine supplements tablets are not known to lead to toxicity when taken at the recommended dosage.²

That said, certain side effects can occur. Common mild reactions include nausea, a flushed feeling, sweating, itching or mild gastrointestinal discomfort.³ For example, one review of side‑effects lists flushing, nausea, sweating and itching, particularly at higher doses or when given by injection.³ Another source reports that minor reactions such as rashes or itchiness, or even injection‑site reactions, have occurred.⁵

More serious reactions are very rare but have been documented. For instance, anaphylactic reactions following intravenous thiamine are noted in very few cases.² However, intravenous thiamine is only given under medical supervision, so allergic reactions can be dealt with promptly if they occur. The UK’s National Health Service lists possible severe allergic effects -— swelling of the lips or /throat, difficulty breathing and, skin changes —- though these are extremely rare and flagged as needing emergency care.⁶

To put this into perspective: the need to worry about side effects at the usual oral supplement doses is very low for healthy people. However, that doesn’t mean the risk is zero. As with all supplements, individual sensitivities and specific health conditions matter. Also note that many forms of vitamin B1 can be in complex formulations – representing either multivitamins or B‑complex blends – so it’s possible you might experience side effects as a result of other ingredients.

Is it okay to take vitamin B1 daily?

Yes – for the great majority of adults, an intake of appropriate amounts daily is not only OK but essential because B1 is used for energy metabolism constantly in the body. Stores are limited, so a regular supply either from diet or, when needed, supplements makes sense.¹ Because it is a water-soluble vitamin, “overflow” beyond what the body needs tends to be excreted rather than to be stored in toxic amounts.²

Most healthy adults can fulfill their vitamin B1 requirements through diet alone. Food sources include whole grains, legumes, pork, fish and fortified foods containing B1.¹ 

If you choose to supplement your diet, keeping within recommended ranges is a good idea. Although no tolerable upper limit is determined for oral thiamine in most healthy people (no reported adverse effects even at several hundred mg/day in oral form), it remains good practice not to take large doses without medical supervision.

Daily low‑to‑moderate supplementation (e.g., a B‑complex that supplies B1 alongside other B vitamins) is often preferable to high single-vitamin doses. The idea is maintaining supply rather than pushing high amounts. And always discuss with your healthcare provider if you have any underlying health conditions or take medications that could interact with vitamin B1.

Should I take vitamin B1?

Here’s a pros‑and‑cons wrap‑up to help you decide whether supplementation with vitamin B1 makes sense for you.

Potential benefits

  • Daily intake supports normal energy conversion, nervous system function and carbohydrate metabolism.
  • Because deficiency is rare in populations with access to a balanced diet, the risk of missing out is low, but certain groups (e.g., those with malabsorption, heavy alcohol use or very restricted diets) may benefit more.

Things to consider

  • Most people get enough vitamin B1 from their diet alone. Unnecessary supplementation can sometimes lead to choosing more complex products, such as B-complex or multivitamin formulas, which contain multiple nutrients rather than vitamin B1 alone. This may result in consuming vitamins or minerals that are not needed and can make it harder to identify the cause of any benefit or side effect.
  • While side‑effects are rare and mild in most cases, every supplement comes with a small risk of allergic reaction or interaction.
  • If you have medical conditions (kidney or liver problems), take medications or have a history of allergic responses, you should consult your healthcare provider as there may be additional risks.
  • Because many B1 supplements come as part of a multivitamin or complex, the other ingredients also have the potential to drive unwanted side effects.

In short: if you are healthy, eat a reasonably varied diet and do not fall into a high‑risk category, you likely do not need to worry about side effects from standard doses of vitamin B1. However, you can always consult with a doctor or pharmacist if you’re unsure. 

As to the question of whether you need B1 supplements: if you are someone with malabsorption issues or restrictive dietary patterns, supplementation might be worth discussing with a healthcare professional. But for many, it’s simple enough to eat a diet rich in B1.

References 

  1. Martel, J. L. (2024). Vitamin B1 (Thiamine). In StatPearls. NCBI Bookshelf. https://www.ncbi.nlm.nih.gov/books/NBK482360/
  2. Office of Dietary Supplements. (2023, February 9). Thiamin (vitamin B1) fact sheet for health professionals. National Institutes of Health. https://ods.od.nih.gov/factsheets/Thiamin-HealthProfessional/
  3. National Health Service. (2023, February 9). Thiamine (vitamin B1) – Side effects of thiamine. https://www.nhs.uk/medicines/thiamine-vitamin-b1/side-effects-of-thiamine/
  4. SingleCare. (2025, April 30). Thiamine side effects and how to avoid them. https://www.singlecare.com/blog/thiamine-side-effects/
  5. European Commission, Scientific Committee on Food. (2001, July 16). Opinion of the SCF on the tolerable upper intake level of Thiamine. https://food.ec.europa.eu/system/files/2020-12/sci-com_scf_out93_en.pdf
  6. Drugs.com. (2024, November 25). Thiamine – Uses, side effects & warnings. https://www.drugs.com/mtm/thiamine.html