The oak tree, respected for millennia as a source of strong, dense wood, also has a considerable tradition of medicinal use. The astringent, tannin-rich bark of the oak tree has been recommended for such diverse conditions as internal hemorrhage, diarrhea, dysentery, cancer, and pneumonia.

Currently, Germany’s Commission E recommends oak bark internally for treatment of diarrhea and topically for sore throat, mouth sores, hemorrhoids, and eczema. However, there is no meaningful scientific evidence that oak bark offers any therapeutic benefit in these or any other conditions. Onlydouble-blind, placebo-controlled studies can prove a treatment effective, and none have been performed on oak bark. (For more information on why such studies are essential, see Why Does This Database Rely on Double-blind Studies?)

Oak bark contains numerous substances in the tannin family, especially ellagitannin,1along with potentially active substances in the saponin family.2 Tannins are thought to have an astringent effect, meaning that they reduce tissue swelling and stop bleeding, and they are traditionally thought to be useful for diarrhea. However, oak bark has never been studied as a treatment for diarrhea. Saponins are often said to act as expectorants, enhancing the ability to cough up phlegm. Again, however, there is no direct evidence that oak bark is useful for coughs or related conditions.

Very weak evidence (too weak to be relied upon at all) hints that oak bark may have value for kidney stones, possibly reducing pain and slowing stone growth.3 In addition, test-tube studiesindicate that oak bark solutions applied topically might have activity against various microorganisms, including staphylococcus,4-6 and might also exert cancer-preventiveeffects.7 However, it is a long way from such studies to actual evidence of clinical benefit.