The U.S. Food and Drug Administration does not strictly regulate herbs and supplements. There is no guarantee of strength, purity or safety of products, and effects may vary. You should always read product labels. If you have a medical condition, or are taking other drugs, herbs, or supplements, you should speak with a qualified healthcare provider before starting a new therapy. Consult a healthcare provider immediately if you experience side effects.
Rare hypersensitivity or allergic reactions have occurred with thiamine supplementation. A small number of life-threatening anaphylactic reactions have been observed with large parenteral (intravenous, intramuscular, subcutaneous) doses of thiamine, generally after multiple doses.
Skin irritation, burning, or itching may rarely occur at injection sites.
Contact dermatitis may occur with occupational exposure and may cause sensitization and lead to dermatitis-type reactions after subsequent oral or injected administrations.
Side Effects and Warnings
Thiamine is generally considered safe and relatively nontoxic, even at high doses. No clear tolerable upper level (UL) of intake has been established. Dermatitis or more serious hypersensitivity reactions occur rarely. Large doses may cause drowsiness or muscle relaxation.
Injections of thiamine may cause burning. Reactions can often be avoided by slow administration into larger veins.
Supplementary thiamine use in cancer patients should be under the care of a physician.
Use cautiously in patients with low blood pressure or in those taking blood pressure-lowering agents, as in an elderly population with subclinical thiamine deficiency, thiamine supplementation resulted in decreased systolic blood pressure.
Use cautiously in patients with a slow heart rate, as, in human research, thiamine pyrophosphate (TPP) has been shown to decrease heart rate.
Use cautiously with vasodilators (blood vessel-widening agents) in patients with high blood sugar levels or diabetes, as thiamine has been shown to improve vasodilation in these patients.
Avoid use of thiamine at levels higher than those commonly found in available marketed products, unless under the advice of a healthcare practitioner.
Avoid in patients with known allergy or hypersensitivity to any constituents in thiamine supplements.
Pregnancy and Breastfeeding
Thiamine is classified by the U.S. Food and Drug Administration (FDA) as Pregnancy Category C. Thiamine may be recommended by a healthcare practitioner in women who are vomiting excessively during pregnancy.