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 studies in pregnant women have not shown more fetal abnormalities but cannot rule out harm. The documented problem in pregnancy is too little thiamine, with severe vomiting. Ask your doctor first.^1,2,3,4

Key facts

NIH says thiamin has not been shown to cause any harm, and the Food and Nutrition Board set no upper limit because no harm from high intakes has been reported.^1,2
The label of a thiamine injection says studies in pregnant women have not shown that thiamine increases the risk of fetal abnormalities, but cannot rule out harm.³
The same label lists neuritis of pregnancy with severe vomiting among the reasons to give thiamine by injection.³
A systematic review of 177 reported cases found that women with severe vomiting in pregnancy became depleted of thiamine between 10 and 15 weeks of gestation.⁴
In that review, 63.6% of the women had been treated with too little thiamine.⁴

What does NIH say about thiamin and harm?

NIH's consumer fact sheet is short on the question of harm: thiamin has not been shown to cause any harm. The professional sheet gives the reason there is no upper limit. The Food and Nutrition Board found no reports of adverse effects from high intakes of 50 mg a day or more from food or supplements, and the body absorbs much less above about 5 mg a day and passes the excess in urine.^1,2

NIH reports no adverse effects from high thiamin intakes of 50 mg a day or more from food or supplements, although experts note that very large intakes could still have effects.^1,2

The board also noted that, despite the lack of reports, very large intakes could still have effects. NIH's sheets do not give a separate caution for pregnancy.²

What does a thiamine injection label say about pregnancy?

The label of a thiamine injection has a section on pregnancy. It says studies in pregnant women have not shown that thiamine hydrochloride increases the risk of fetal abnormalities when given during pregnancy, and that the possibility of fetal harm appears remote. It then adds that because studies cannot rule out harm, the injection should be used during pregnancy only if clearly needed.³

The same label lists, among the conditions for which rapid replacement of thiamine is needed, neuritis of pregnancy if vomiting is severe. That is a prescription injection used under medical care, not a supplement.³

Why is thiamine discussed in pregnancy at all?

The documented problem in the sources is too little thiamine, not too much. A 2019 systematic review of Wernicke's encephalopathy in women with hyperemesis gravidarum, the severe form of vomiting in pregnancy, gathered 146 case studies covering 177 cases. Women became thiamine depleted between 10 and 15 weeks, after vomiting for a median of 7 weeks and losing 12.1 kg.⁴

Warning signs before the condition were nausea and vomiting in all cases, double vision in 37.4% and blurred vision in 27.4%. Treatment with too little thiamine was common, in 63.6% of cases, and the authors reported chronic cognitive problems in 65.4% of cases, pregnancy loss in 50% and maternal death in 5%. Case reports tend to describe the most severe cases, so these are not rates for all women with vomiting in pregnancy.⁴

The authors concluded that thiamine supplementation was insufficient or absent from treatment plans, and called for thiamine to be given by injection to women with persistent or severe late-onset vomiting. Those are decisions for a doctor, and the review's point is that severe vomiting needs medical attention.^2,4

What do the sources not cover?

The sources checked do not report trials of thiamin supplements in healthy pregnant women, and they do not address vitamin B1 in prenatal vitamins separately. That is not a finding either way, so ask a doctor or pharmacist about any supplement that is not covered here.^2,3

Who should ask a doctor about thiamine in pregnancy?

Anyone who is vomiting frequently or for a long time in pregnancy, since the systematic review found thiamine can run out by 10 to 15 weeks.⁴
Anyone who is pregnant and drinks heavily or is recovering from alcohol dependence, the most common cause of thiamin deficiency in wealthy countries.²
Anyone who has had bariatric surgery, which NIH says can cause severe thiamin deficiency.²
Anyone who takes furosemide or another loop diuretic.²

Questions people ask

Is vitamin B1 safe during pregnancy?

NIH says thiamin, also called vitamin B1, has not been shown to cause any harm. A thiamine injection label says studies in pregnant women have not shown more fetal abnormalities but cannot rule out harm. Ask your doctor.^1,3

Is thiamine safe in pregnancy with severe vomiting?

A 2019 systematic review of 177 cases found women with severe vomiting can become depleted of thiamine by 10 to 15 weeks, and that treatment with too little thiamine was common. Ask your doctor promptly if vomiting is severe or lasting.⁴

Is there an upper limit for thiamine in pregnancy?

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

Is thiamine in a prenatal vitamin safe?

The sources checked do not address thiamin in prenatal vitamins separately. NIH says thiamin has not been shown to cause harm. Ask your doctor or pharmacist about your prenatal vitamin.^1,2

See what we make

Every product shows its whole label and what its lab report covers. The store opens soon.

See the Full Measure line

Sources

This page is information, not medical advice. Talk to your doctor or pharmacist about your own situation.