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[Didang Pill Formula]
Shuizhi (Leech, *Hirudo nipponica*) (stir-fried) 20 pieces, Mengchong (Blue Bottle Fly Larva, *Calliphora vicina* larva) (stir-fried, wings and legs removed) 20 pieces, Tao Ren (Peach Kernel, *Prunus persica* seed) (skin and tip removed) 25 pieces, Dahuang (Rhubarb Root, *Rheum palmatum* root) 3 liang.
The above four ingredients are crushed, sifted, and formed into four pills. Boil one pill in one sheng of water, reduce to seven ge, and administer. Within a day and night, bloody stool should be passed; if not passed, administer again.

[Collected Commentary]
Ke Qin stated: The Bladder is a fu-organ, and blood has no place to accumulate within it. The Shaoyang abdomen (lower abdomen) is the residence of the Bladder. When heat binds causing fullness, urinary difficulty should occur; however, if urination is actually free, it indicates that the disease is not inside the Bladder but within the lower abdomen. The ying-blood traveling along the meridians, due to stasis-heat binding in the interior of the lower abdomen, rather than the interior of the Bladder itself. Therefore, although urination is free, the fullness, urgency, and binding sensation in the lower abdomen remain unchanged, indicating blood stasis accumulating in the lower abdomen. Heat overflowing internally disturbs the shen-spirit, leading to mania; when blood is stagnant and fails to circulate, ying-nutrition cannot transport, resulting in a faint and deep pulse; when ying-nutrition fails to circulate, qi mechanism cannot diffuse, resulting in a deep and knotted pulse. When ying-qi does not circulate throughout the body, jaundice occurs. Excessive digestion with frequent hunger indicates blazing stomach-fire; bowel movements being surprisingly easy to pass indicates lubrication by blood; black stool color indicates transformation of accumulated blood; forgetfulness indicates that blood fails to nourish intelligence, leading to unclear spirit. These are all signs of blood stasis. Only extremely potent medicines are sufficient to attack their nest and shoulder this heavy responsibility; thus, Didang Decoction was established. Leeches are good at drinking blood and thrive in water; fly larvae are good at sucking blood and are fierce on land. By combining these creatures from both water and land that excel at extracting blood, we seek similarity to attract and attack [the stasis], following the principle of "like attracts like." Furthermore, assisted by the bitter-warm nature of Peach Kernels to push out the old and generate the new, and the bitter-cold nature of Rhubarb to sweep away pathogenic heat, it is named Didang. If the heat is severe but has not reached the point of mania, and the lower abdomen is full but not yet urgent and bound, the dosage should be reduced and formulated into pills for slow treatment. If exterior symptoms have resolved, but there is urgency and binding in the lower abdomen, and the patient exhibits manic behavior, this is a syndrome transferring to the Yangming stage. Use Tiao Wei Cheng Qi Decoction added with blood-moving agents such as Peach Kernels and Cinnamon Twig, to induce mild purgation, so that when the Stomach harmonizes, recovery occurs. This explains why Taohao Cheng Qi Decoction serves as a milder treatment method.

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After severe purgation in Cold Damage, followed by repeated sweating, if epigastric stuffiness (pi) occurs accompanied by aversion to cold, the exterior is unresolved. One must not attack the pi directly; instead, resolve the exterior first. Only after the exterior is resolved can the pi be attacked. For resolving the exterior, Guizhi Tang (Cinnamon Twig Decoction) is suitable; for attacking the pi, Dahuang Huanglian Xiexin Tang (Rhubarb and Coptis Heart-Purging Decoction) is suitable.

[Annotation]
In Cold Damage, after severe purgation followed by repeated sweating, the sequence of treatment—purgation first, then sweating—is incorrect. Pathogenic heat sinks inward, causing epigastric pi-binding, which legally requires attacking the interior. However, if aversion to cold is present, it indicates that the exterior pathogen is not fully expelled. Since the exterior is not fully resolved, one must not attack the pi; instead, resolve the exterior first. Only after the exterior is resolved can the pi be attacked. Guizhi Tang (Cinnamon Twig Decoction) is suitable for resolving the exterior because this is an exterior condition existing after prior sweating and purgation. Dahuang Huanglian Xiexin Tang (Rhubarb and Coptis Heart-Purging Decoction) is used to attack the pi because it addresses pi caused by interior heat after the exterior has resolved.

[Collected Notes]
*Huoren Shu* (Book of Vitality) states: Generally, Jie Xiong (Colliding Chest) and Pi (Stuffiness) syndromes should be treated with purgation; however, if the exterior is unresolved, purgation must not be used.
Fang Youzhi stated: The "exterior" here refers not to the initial disease exterior, but to the exterior loosened by subsequent sweating after purgation. "Resolve" means to relieve or save, similar to relieving thirst or emergency. Resolving the exterior differs from releasing the exterior. For the initial exterior of Cold Damage, one should release it using Mahuang Tang (Ephedra Decoction); here, since it is the post-sweating exterior, it should be relieved, hence stating suitability for Guizhi Tang (Cinnamon Twig Decoction).
Zhang Lu stated: Severe purgation followed by sweating represents treating the interior before the exterior, an inverted error. Ultimately, the sunk pathogen binds in the epigastrium, and the syndrome includes aversion to cold. Since the exterior pathogen does not diminish with sweating, one must not further attack the pi. Instead, one should first employ muscle-releasing methods to treat the exterior. Only after the exterior is resolved should Dahuang Huanglian Xiexin Tang (Rhubarb and Coptis Heart-Purging Decoction) be used to attack the pi condensed by pathogenic heat. This constitutes the lawful approach.