If you are currently being treated with any of the following medications, you should not use vitamin B1 without first talking to your healthcare provider.

Antibiotics, Tetracycline
Vitamin B1 should not be taken at the same time as the antibiotic tetracycline because it interferes with the absorption and effectiveness of this medication. Vitamin B1 either alone or in combination with other B vitamins should be taken at different times from tetracycline. (All vitamin B complex supplements act in this way and should therefore be taken at different times from tetracycline.)

Antidepressant Medications, Tricylic
Taking vitamin B1 supplements may improve treatment with antidepressants such as nortriptyline, especially in elderly patients. Other medications in this class of antidepressants include desimpramine and imipramine.

Chemotherapy
Although the significance is not entirely clear, laboratory studies suggest that thiamine may inhibit the anti-cancer activity of chemotherapy agents. How this will ultimately prove relevant to people is not known. However, it may be wise for people undergoing chemotherapy for cancer to not take large doses of vitamin B1 supplements.

Digoxin
Laboratory studies suggest that digoxin (a medication used to treat heart conditions) may reduce the ability of heart cells to absorb and use vitamin B1; this may be particularly true when digoxin is combined with furosemide (a loop diuretic).

Diuretics
Diuretics (particularly furosemide, which belongs to a class called loop diuretics) may reduce the levels of vitamin B1 in the body. In addition, similar to digoxin, furosemide may diminish the heart's ability to absorb and utilize vitamin B1, especially when these two medications are combined.

Scopolamine
Vitamin B1 may help reduce some of the side effects associated with scopolamine, a medication commonly used to treat motion sickness.