Tannic acid is safe when used in the amounts found in foods. However, it seems unsafe when applied to the skin or when taken by mouth as a medicine. In large amounts, it can cause side effects such as stomach irritation, nausea, vomiting, and liver damage.
Do not use tannic acid if:
You are pregnant or breast-feeding.
You have a kidney or liver disorder.
You have certain skin conditions.
You have a fever or infection.
You have heart problems.