Vitamin B1 tablets, or thiamine, should not normally be accompanied by any side effects, according to the National Institute's of Health's MedlinePlus. The benefits of any vitamin therapy should outweigh the risks and side effects and if any adverse reactions are apparent, the patient should immediately seek the assistance of a health care provider.
Thiamine should be taken with caution for patients with low blood pressure, diabetes, or allergies or sensitivity to thiamine, advises the Mayo Clinic website. The United States Food and Drug Administration does not strictly regulate vitamin supplements. Consumers should read labels carefully, consult a physician and be aware of any drug interactions when taking supplements of any kind.