Quick answer
NIH says thiamin has not been shown to cause any harm, and no upper limit is set. The label of a thiamine injection says it is not known whether the drug is excreted in human milk. A trial in 335 nursing mothers found supplements raised the thiamine in milk but did not report safety results. Ask your doctor first.1,2,3,4
Key facts
- NIH says thiamin has not been shown to cause any harm, and no upper limit has been set because no harm from high intakes has been reported.1,2
- The label of a thiamine injection says it is not known whether the drug is excreted in human milk and advises caution in a nursing mother.3
- A double-blind trial in 335 Cambodian mothers found thiamine capsules raised the thiamine in breast milk compared with placebo.4
- In that trial, 1.2 mg a day was enough to raise milk thiamine to levels similar to those with 2.4 or 10 mg a day.4
- The trial's abstract reports milk and blood thiamine levels and does not report safety outcomes.4
What does NIH say about thiamin during lactation?
NIH's fact sheets list a recommended thiamin intake for breastfeeding teens and women that is higher than for other adults, and they say thiamin has not been shown to cause any harm. The professional sheet explains that no upper limit was set because high intakes of 50 mg a day or more have produced no reported adverse effects, and that absorption falls above about 5 mg a day.1,2
A mother's thiamin intake changes the thiamin in her breast milk very little unless she is short of it: milk from mothers taking 1.7 mg a day in a multivitamin had about 10% more.5
Breast milk carries about 0.15 mg of thiamin a day to a baby who is only breastfed in the first six months.5
What does a thiamine injection label say about nursing?
The label of a thiamine injection says it is not known whether the drug is excreted in human milk, and that because many drugs are, caution should be exercised when it is given to a nursing mother. That sentence is standard wording on drug labels, and it concerns an injection given under medical care. It is not a finding that thiamine harms a nursing baby.3
The same label includes infantile beriberi with acute collapse among the conditions where rapid thiamine replacement is needed, which shows the concern in infants is deficiency.3
What did the trial in nursing mothers find?
A 2021 double-blind randomized trial in Cambodia looked at whether extra thiamine raises the thiamine in breast milk, where beriberi in infants is still a problem. It gave 335 healthy mothers a capsule with 0, 1.2, 2.4 or 10 mg of thiamine a day from 2 weeks to 24 weeks after birth.4
| Group | Mothers assigned | Milk thiamine |
|---|---|---|
| Placebo | 83 | 153 mcg a liter |
| 1.2 mg a day | 86 | 183 mcg a liter |
| 2.4 mg a day | 81 | 190 mcg a liter |
| 10 mg a day | 85 | 206 mcg a liter |
Milk thiamine was significantly higher in all three groups than with placebo, and the three did not differ from each other. The abstract reports no safety findings, so the trial shows what reaches the milk and not whether any amount carries a risk.4
Who should ask a doctor about thiamine while breastfeeding?
- Anyone who drinks heavily or is recovering from alcohol dependence, since NIH says it is the most common cause of thiamin deficiency in wealthy countries.2
- Anyone who has had bariatric surgery, who NIH says can develop severe thiamin deficiency.2
- Anyone who takes furosemide or another loop diuretic, or is having chemotherapy with fluorouracil.2
- A parent whose baby has poor feeding, weakness or other worrying signs, who should ask the baby's doctor and mention the mother's diet and any supplements.3,4
Questions people ask
Is vitamin B1 safe while breastfeeding?
NIH says thiamin, also called vitamin B1, has not been shown to cause any harm. A thiamine injection label says it is not known whether the drug is excreted in human milk. Ask your doctor.1,3
Does thiamine get into breast milk?
Yes. NIH says breast milk carries thiamin to a baby, and a 2021 trial found supplements raised milk thiamine in nursing mothers compared with placebo. A mother's own intake changes it little unless she is short of it.2,4
Is thiamine safe for a breastfed baby?
The sources checked do not report harm to breastfed babies from a mother's thiamine, and they do not study the question directly. Ask your baby's doctor if you have concerns.2,4
Is there an upper limit for thiamine while breastfeeding?
No. The Food and Nutrition Board set no upper limit for thiamin because no harm has been reported from high intakes from food or supplements.2
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Sources
- Office of Dietary Supplements, National Institutes of Health. Thiamin: Fact Sheet for Consumers. Updated March 22, 2021. Opened 2026-10-07.
- National Institutes of Health, Office of Dietary Supplements. Thiamin: Fact Sheet for Health Professionals. Updated February 9, 2023. Opened 2026-10-07.
- Thiamine hydrochloride injection: prescribing information (Lifestar Pharma). DailyMed, U.S. National Library of Medicine. Published October 1, 2026. Opened 2026-10-07.
- Gallant J, Chan K, Green TJ, et al. Low-dose thiamine supplementation of lactating Cambodian mothers improves human milk thiamine concentrations: a randomized controlled trial. Am J Clin Nutr. 2021;114(1):90-100. Opened 2026-10-11.
- EFSA Panel on Dietetic Products, Nutrition and Allergies (NDA). Dietary reference values for thiamin. EFSA J. 2016;14(12):e04653. Opened 2026-10-07.
This page is information, not medical advice. Talk to your doctor or pharmacist about your own situation.