Quick answer

Thiamine taken by mouth reaches its highest level in the blood within about 1 to 4 hours in studies. Refilling low body stores takes much longer: NIH cites a plan of 10 mg a day for a week, then 3 to 5 mg a day for at least six weeks.^1,2,3

Key facts

In two studies, thiamin taken by mouth peaked in the blood between about 1 and 4 hours later.^1,2
Thiamin's biological half-life in the body is about 9 to 18 days.⁴
With no thiamin at all in the diet, body stores can run down in about three weeks.⁵
An adult body holds only about 25 to 30 mg of thiamin.³

Hours: getting into the blood

In 6 healthy adults, a 50 mg dose by mouth reached its highest level in blood plasma after about 53 minutes, with an elimination half-life of about 2.5 hours.¹

Taken an hour after breakfast, single doses of 100 to 1,500 mg peaked in the blood about 3 to 4 hours later.²

In the 50 mg study, only 2.5% of the dose turned up in urine over the next day, a sign of how little was absorbed. Larger doses push more through the gut wall, which is why 1,500 mg produced far higher blood levels than 100 mg.^1,2

Days to weeks: how long it stays

Blood levels and body stores tell different stories. A single dose clears from plasma within hours, while the thiamin already inside cells, mostly as thiamin diphosphate, lasts much longer. After an injection, free thiamin had a half-life of about 1.6 hours in plasma, but thiamin diphosphate in the blood had a half-life of about 11 hours.^1,4

Once inside the tissues, thiamin turns over slowly: its biological half-life in the body is about 9 to 18 days.⁴

Body stores can run down after about three weeks with no thiamin at all in the diet.⁵

When intake is far above need, tissue stores rise only two to three times, and the extra leaves in urine.^3,5

Will you notice anything?

If you already get enough thiamin, probably not. The extra is absorbed less efficiently and passed in urine, and the studies on this page measured blood and urine levels, not how people felt.^3,4

Because the body holds only 25 to 30 mg and turns it over within weeks, the useful habit is a steady daily intake from food or a supplement rather than an occasional large dose.^3,4

If your thiamin is low

For a mild shortfall, the World Health Organization plan cited by NIH uses 10 mg of thiamin a day by mouth for a week, then 3 to 5 mg a day for at least six weeks.³

After injections for a deficiency, the injectable thiamin label recommends a multivitamin with 5 to 10 mg of thiamin daily for a month to saturate the tissues. Severe deficiency is treated in hospital, often with injections, because thiamin by mouth may not raise blood levels enough.^3,5

Confusion, unsteady walking, or tingling and numbness in the hands and feet need a doctor, not a supplement plan, especially in someone who drinks heavily.^3,6

Questions people ask

How long does thiamine stay in your system?

A single dose leaves the blood plasma within hours: in one study its elimination half-life was about 2.5 hours. The thiamin held in tissues lasts much longer, with a biological half-life of about 9 to 18 days.^1,4

How long should you take thiamine?

Healthy people need a little every day, ideally from food, so there is no set course. When a doctor corrects a mild shortfall, the plan NIH cites runs for at least seven weeks in total.³

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Sources

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