Vitamin B1 Safety

Toxicity

The Food and Nutrition Board did not set a tolerable upper level (UL) of intake for thiamin because there are no known toxic effects from the consumption of excess thiamin in food or through long-term oral supplementation (up to 200 mg/day). A small number of life threatening anaphylactic reactions have been observed with large intravenous doses of thiamin. However, anaphylactic reactions are the result of an overwhelming allergic response rather than a toxic effect of thiamin.

Drug interactions

Reduced blood levels of thiamin have been reported in individuals with seizure disorders (epilepsy) taking the anticonvulsant medication, phenytoin, for long periods of time. 5-Fluorouracil, a drug used in cancer therapy, inhibits the phosphorylation of thiamin to thiamin pyrophosphate (TPP). Diuretics, especially furosemide (Lasix), may increase the risk of thiamin deficiency in individuals with marginal thiamin intake due to increased urinary excretion of thiamin.

Older adults (65 years and older)

Presently, there is no evidence that the requirement for thiamin is increased in older adults, but some studies have found inadequate dietary intake and thiamin insufficiency to be more common in elderly populations. Thus, it would be prudent for older adults to take a multivitamin/multimineral supplement, which will generally provide at least 1.5 mg of thiamin/day.