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Treats cases where, after five or six days of Taiyang cold damage (Taiyang Cold Damage), there is alternating chills and fever (alternating chills and fever), bitter fullness in the chest and hypochondrium (bitter fullness in the chest and hypochondrium), silence with loss of appetite (silence with loss of appetite), irritability and frequent vomiting (irritability and frequent vomiting), bitter taste in the mouth and deafness (bitter taste in the mouth and deafness), and a wiry and rapid pulse (wiry and rapid pulse). This is a syndrome of the Shaoyang meridian at the half-exterior and half-interior level. It is appropriate to use this decoction to harmonize the exterior and interior.

Chai Hu (Bupleuri Radix) 8 liang, Huang Qin (Scutellariae Radix) 3 liang, Ren Shen (Ginseng Radix et Rhizoma) 3 liang, Ban Xia (Pinelliae Rhizoma) half sheng, Zhi Gan Cao (Prepared Glycyrrhizae Radix et Rhizoma) 3 liang, Sheng Jiang (Zingiberis Rhizoma Recens) sliced 3 liang, Da Zao (Jujubae Fructus) split 12 pieces. The above seven ingredients are boiled in 12 sheng of water, reduced to 6 sheng, filtered to remove dregs, and then boiled again to obtain 3 sheng. Take 1 sheng warm, three times daily. If there is vexation in the chest but no vomiting, remove Ban Xia and Ren Shen, add Gua Lou Shi (Trichosanthis Fructus). If there is thirst, remove Ban Xia, increase the dose of Ren Shen, and add Gua Lou Gen (Trichosanthis Radix). If there is abdominal pain, remove Huang Qin, add Bai Shao (Paeoniae Radix Alba). If there is a feeling of stuffiness and hardness under the costal margin, remove Da Zao, add Mu Li (Ostreae Concha). If there is palpitation in the epigastrium and difficult urination, remove Huang Qin, add Fu Ling (Poria). If there is no thirst but slight external heat, increase the dose of Ren Shen and add Gui Zhi (Cinnamomi Ramulus), cover with blankets to induce slight sweating for recovery. If there is coughing, remove Ren Shen, Da Zao, and Sheng Jiang, add Wu Wei Zi (Schisandrae Chinensis Fructus) and Gan Jiang (Zingiberis Rhizoma).

[Collected Notes] Cheng Yingmao said: The formula is named Xiao Chai Hu Tang (Minor Bupleurum Decoction) because it corresponds to the Shaoyang meridian. As for the principles of formulation and modification methods, they encompass the aforementioned concepts: when the Upper Jiao (Upper Burner) is unobstructed, body fluids descend, and the Stomach Qi harmonizes. Why? The Shaoyang meridian runs along the hypochondrium, situated between the Yang aspect of the abdomen and the Yin aspect of the back. When pathogenic factors from the Exterior attempt to enter the Interior, they are resisted by the Interior Qi, resulting in alternating chills and heat. Because Exterior and Interior resist each other and remain at the boundary, there is bitter fullness in the chest and hypochondrium. Due to resistance, mental clarity is obscured, leading to silence. When Wood Qi is affected, it impairs Earth Qi (Spleen and Stomach), leading to loss of appetite. The Gallbladder is Yang Wood residing in clear pathways; when constrained by pathogens, Fire has no outlet, forcing it upward to disturb the Heart, causing irritability. When Clear Qi stagnates and becomes turbid, phlegm forms, leading to vomiting; vomiting releases the constraint on Wood and Fire, hence the patient feels relief ("likes" vomiting). These are fixed symptoms of the Shaoyang meridian. The other symptoms mentioned with "or" occur because Shaoyang is a wandering sector in the human body; wherever there are gaps in the meridians of the Exterior and Interior, symptoms can appear according to deficiency, representing unpredictable pathogenic manifestations. Although these symptoms may also appear in Taiyang disease, their appearance on the fifth or sixth day classifies them as Shaoyang, possessing both half-exterior and half-interior characteristics, which constitutes the indication for Xiao Chai Hu Tang. In the formula, Chai Hu soothes Wood Qi, allowing half-exterior pathogens to be dispersed outward; Huang Qin clears Fire, allowing half-interior pathogens to be resolved inward; Ban Xia resolves phlegm-fluids and directs rebellious Interior Qi downward; Ren Shen tonifies chronic deficiency and assists the generation of ascending Qi; Gan Cao assists Chai Hu and Huang Qin to harmonize the Interior and Exterior; Sheng Jiang and Da Zao assist Ren Shen and Ban Xia to circulate Ying and Wei Qi. These herbs support and complement each other, preventing pathogens from moving inward and facilitating their resolution outward. Regarding vexation without vomiting, Fire has become dry and solid, pressing against the chest; thus, Ren Shen and Ban Xia are removed, and Gua Lou Shi is added. Regarding thirst, Dryness has consumed fluids and pressed against the Lung; thus, Ban Xia is removed, and Gua Lou Gen is added. Regarding abdominal pain, Wood Qi scatters into Earth, confining Stomach Yang; thus, Huang Qin is removed to calm Earth, and Bai Shao is added to restrain Wood. Regarding stuffiness and hardness under the costal margin, since pathogens have lingered, Wood Qi is excessive; thus, the cloying sweetness of Da Zao is removed, and the salty softening property of Mu Li is added. Regarding palpitation and difficult urination, Water Pathogens invade the Heart; thus, the bitter-cold draining nature of Huang Qin is removed, and the bland leaching property of Fu Ling is added. Regarding no thirst but slight muscle heat, Cold from the half-exterior still lingers in the muscles; thus, Ren Shen is removed, and Gui Zhi is added to resolve the exterior. Regarding coughing, Cold from the half-exterior gathers in the Lung; thus, Ren Shen and Da Zao are removed, Wu Wei Zi is added, and Sheng Jiang is replaced with Gan Jiang to warm the Lung; although the Lung is cold, Huang Qin is not reduced, lest Gan Jiang aid Heat. In summary, when pathogens are in the Shaoyang, presenting as Exterior Cold and Interior Heat, both constrained and unable to ascend, the treatment of Xiao Chai Hu Tang follows the principle of conforming to the ascending, descending, floating, and sinking movements.