Tannic acid is a naturally occurring substance and is not usually harmful to human health in and of itself. In fact, it is frequently used as an approved additive in a variety of foods and beverages. However, you should be careful about drinking water that contains additives with unknown sources -- even if they initially seem harmless.
Tannic acid is in a class of chemicals called tannins, which are found in many plants. Tannic acid refers to tannins that can be dissolved in water, as opposed to the other type, or condensed tannins. According to Cornell University Department of Animal Science, tannins are a group of compounds known as plant polyphenols, which cause proteins to bind and form complexes. Tannins are found in many of the foods you eat, including fruits, teas, wine and edible grasses -- like corn.
Tannins can occur in drinking water, usually from harmless natural sources like leaves and tree bark. While tannic acid is generally safe to consume in small amounts, it can impart a yellowish color and bitter taste to your water. Furthermore, if your home water source is well water, the presence of tannic acid could indicate contamination from other substances in your water. Even though the tannic acid itself is not usually harmful, its presence in drinking water indicates the possibility that other, more harmful additives have also entered your water. In this case, you should have your well inspected by a water quality professional to determine the safety of your drinking water.
Tannic acid may help reduce your risk of cancer when consumed. According to the National Institutes of Health, tannic acid can have an antioxidant effect on your body. Antioxidants help protect your cells from free radicals, which are damaging substances that can lead to cancer and other chronic diseases. They also help protect your cells from mutagens and carcinogens, which also contribute to cell damage and the occurrence of cancer.
Tannic acid has other health benefits due to its anti-microbial properties. The tannins in fruit seem to naturally protect fruit from the growth of microbes. Tannic acid helps inhibit foodborne bacteria and the growth of certain fungi, yeasts and viruses. This may have applications in the food industry, to help naturally preserve foods. In addition, tannic acid has some medical applications, like accelerating blood clotting and reducing blood pressure.