A tannin is an astringent chemical derived from plants. Tannic acid is one type of tannin that has a fairly weak acidity. In some trees, this chemical may act as protection against pests and fires. It is believed that humans can benefit from the antibacterial, antifungal, and antioxidant properties of the substance. Tannic acid is also used for industrial purposes such as leather production and wood staining.

This substance is usually found as a yellow, white, or light brown powder. It tends to easily dissolve in water. It generally does not have a smell but the taste is one that can cause a person to pucker.

Since it will cause constipation in humans, tannic acid can be used to treat diarrhea. It can also be used to reduce the swelling of hemorrhoids and control internal bleeding. Externally, tannin can be added to creams and salves to help combat muscle and joint problems and to help heal wounds. It may also be used for antifungal treatment of the feet, toenails, or fingernails.

People are warned not to consume large amounts of this substance. It is also regarded as inadvisable to consume the substance on a regular basis. Although it can be helpful in many ways, tannin may also have adverse effects.

One disadvantage of tannic acid is that it is believed to interfere with iron absorption. Iron is a mineral essential to maintaining proper human health. Another disadvantage is that the substance may also have a negative effect on the digestive process. Use of this substance has been associated with severe liver and kidney damage.

These negative symptoms are associated with using the substance in its pure form or with abnormal use of tannic acid concentrations. People are not regarded as being at risk of adverse effects with normal usage and consumption. For example, tannic acid may be absorbed into wine from the oak barrels it is stored in.

Tannic acid is naturally present in some types of wood such as oak and Sequoia. It is sometimes used to pretreat other types of wood that need to be stained. Tannic acid is also commonly used to tan animal skin to produce leather. It is often stated that tanning was the first use that humans made of the substance.