The bark of the oak tree is used medicinally. There are several species of oak and it is unclear which is most effective. Oak trees grow throughout North America and as far south as the Rio Grande. Some species of oak grow around the world, including in China and the Middle East. Tannic acid derived from oak trees has a long history of application in tanning hides and making ink.
Tannins bind liquids, absorb toxins, and soothe inflamed tissues.
The German Commission E monograph suggests a daily dose of 3 grams of the bark. After cooling, a cloth is dipped into the liquid and applied directly to the rash several times a day. Up to 5 cups of this same solution can be drunk each day in cases of diarrhea . Except for the occasional person reporting an upset stomach or constipation after drinking the tea, oak bark is very safe. There is no known direct problem during pregnancy or lactation, though oak can cause constipation.