By Eric Brand

Bai Tou Weng Tang (Pulsatilla Decoction) is a famous formula for dysentery from the Shang Han Lun.  It primarily treats heat toxin dysentery and is often discussed in comparison with Shao Yao Tang (Peony Decoction), which treats damp-heat dysentery.  Damp-heat and heat toxin dysentery differ in their presentation, and the predominance of blood vs. pus (or mucus) is often an important factor to consider.  Outside the traditional realm of dysentery itself, the TCM disease category of dysentery sometimes fits the clinical presentation of other conditions that present with abdominal pain, tenesmus, and blood and mucus in the stool, such as ulcerative colitis or Chron's.

...

More blood than pus
Abdominal pain
Tenesmus
Red tongue with yellow fur
Slippery, rapid pulse

Bai Tou Weng Tang and Shao Yao Tang both treat dysentery with abdominal pain, tenesmus, and a yellow tongue coating.  However, Bai Tou Weng Tang primarily treats heat toxin falling into the blood aspect, manifesting in heat toxin dysentery with more blood than pus, and a red tongue with yellow fur. It primarily clears heat and resolves toxin while cooling the blood and astringing the dysentery.

By contrast, Shao Yao Tang treats damp-heat obstruction with equal blood and pus and a greasy yellow tongue coat. It harmonizes qi and blood while clearing heat and drying dampness.  It also follows the principles of “treating the unstopped by unstopping” (treating diarrhea with diarrhea), and the principle that “moving blood causes pus in the stool to heal on its own, and regulating qi causes tenesmus to be eliminated on its own.”