Educational information, not medical advice — ask your doctor or midwife.
Common oak (Quercus robur) is a large European tree. The part used as a remedy is the bark of young branches. It is dried, cut small and brewed into a strong, bitter, astringent tea.
The bark is very rich in tannins. Tannins bind to proteins and make surfaces feel tight and dry. Folk healers used this drying effect on skin, in mouth rinses, in sitz baths and for loose stools.
The honest verdict on evidence: human trials are few, small and mostly old. Traditional use across Europe is long and consistent. Long use is not proof that something works, and it is not proof that it is safe.
This page covers the bark. Oak acorns, leaves and oak-derived wood extracts are different products and are not the same thing.
The proposed action is astringency, meaning a drying, tightening effect on tissue.
In test-tube work, oak tannins bind to proteins in the top layer of skin and mucous membrane. This appears to form a thin, sealed layer. In theory that layer reduces weeping, soothes raw tissue and makes it harder for germs to settle.
In laboratory tests, the same tannins slow the growth of several bacteria and reduce markers of inflammation. Whether this happens on human skin or in the human gut at the amounts people actually use has not been shown.
For diarrhoea, the idea is that tannins tighten the gut lining and cut fluid loss. This is a reasonable mechanism, but it rests mainly on lab and animal work rather than on good human trials.
Oak bark is sold in a few simple forms:
- Dried cut bark, loose or in tea bags, for a decoction (a tea simmered rather than steeped) - Liquid extract or tincture in alcohol - Bath additive for sitz baths or partial baths - Ointments, creams and gels for skin - Mouthwash and throat gargle preparations - Lozenges containing bark extract, in some countries
The tea tastes strongly bitter and puckering. Pharmacy-grade bark is graded for tannin content; bark stripped from a tree in a park is not, and may carry mould or spray residue.
Oak bark has a long record in European folk medicine. In Russian tradition it is known as dub, and the bark as dubovaya kora.
Village practice used bark decoctions as gargles for sore gums and throats, as washes for weeping skin, as sitz baths for piles, and by mouth for short bouts of diarrhoea. Tanners used the same bark on hides, and the healers borrowed the logic.
German and central European herbal manuals carried it into the twentieth century. Germany's Commission E later listed the bark for short-term diarrhoea and for mild inflammation of the mouth, throat and anal area. That listing drew heavily on tradition and lab data, not on large modern trials.
Stop and see a doctor if any of these happen:
- Diarrhoea lasts more than two days, or returns quickly - There is blood or black colour in your stools - You have a high fever, severe belly pain or signs of dehydration - You see bleeding from the back passage, even if you think it is piles - Skin gets redder, hotter, more painful or starts to ooze pus - You notice yellow skin or eyes, dark urine, or pain under the right ribs - A rash, swelling of the face or lips, or trouble breathing starts after use; this needs urgent care
Also check with a doctor before use if you take a daily prescription medicine, especially digoxin, lithium or thyroid tablets.
In the European Union, oak bark is a registered traditional herbal medicine for short-term diarrhoea and for mild mouth, throat and anal inflammation. In the United States it is sold as a dietary supplement and is not approved by the FDA to treat any disease. In the United Kingdom it is sold under traditional herbal registration rules.
Evidence grades and traditional-use labels are explained in our methodology. Nothing here is medical advice.
What it is typically used for, usual dose, cautions, and the study behind it where real trial data exists.
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