Buckthorn (Rhamnus cathartica) is a deciduous shrub or small tree in the Rhamnaceae family native to Europe and western Asia. It is a thorny shrub with small greenish flowers, dark berries, and glossy leaves. The aged bark are the parts most often used in traditional preparations.

Buckthorn has a long history in older European herbal practice, especially as a very strong purgative bark. The aged bark was used in carefully measured preparations, but the plant is not a gentle household herb and has always required significant caution.

The berries and fresh bark are especially unsuitable for casual use, and the shrub is also widely known as an invasive plant in parts of North America.