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[Formula Analysis]
This formula is derived from Da Qing Long Tang (Major Blue Dragon Decoction) by substituting Shaoyao (Paeoniae Radix Alba, 芍药) for Xingren (Armeniacae Semen Amarum, 杏仁). Although named Yue Bi Jia Gui Zhi Tang (Yuebi Plus Cinnamon Twig Decoction), it is essentially a modified version of Da Qing Long Tang. The removal of Xingren is due to its acrid and dispersing nature which aids Yang dispersion; the use of Shaoyao is because it enters the Yin aspect and has an astringent and收敛 (astringent) property. By substituting one for the other and modifying the dosage, the therapeutic indication changes. With two parts of Gui Zhi Tang (Cinnamon Twig Decoction) as the main component, it does not induce sweating. Thus, the one part of Yue Bi Tang consists merely of Mahuang (Ephedrae Herba, 麻黄) and Shigao (Gypsum Fibrosum, 石膏), utilizing their acrid-cool properties to assist the two parts of Gui Zhi Tang in harmonizing the exterior and clearing heat from the muscle layer. This implies inducing a slight sweat within the framework of "not inducing sweat," which can be understood from this perspective. It differs from Da Qing Long Tang, where Shigao assists Mahuang as a heavy agent to induce sweating and drive out muscle heat.

[Commentary]
Gui Zhi Er Ma Huang Yi Tang (Two Parts Cinnamon Twig and One Part Ephedra Decoction) treats symptoms resembling malaria, occurring twice daily, where sweating leads to resolution, but without the symptom pattern of "more heat than chills"; therefore, the cooling property of Shigao is not used. Gui Zhi Ma Huang Ge Ban Tang (Equal Parts Cinnamon Twig and Ephedra Decoction) treats symptoms resembling malaria with more heat than chills, yet it does not use Shigao and instead doubles the amount of Mahuang. This is because the patient presents with a flushed facial color indicating stagnant heat and skin itching, indicating that the heat is moving outward toward the exterior rather than inward. Allowing a slight sweat helps follow the pathogenic trend; thus, the cooling interior effect of Shigao is also not used. Gui Zhi Er Yue Bi Yi Tang (Two Parts Cinnamon Twig and One Part Yuebi Decoction) treats fever and aversion to cold with more heat than chills, and uses Shigao because there is little cold on the exterior but much heat in the muscle interior. Therefore, the cooling property of Shigao is used to assist Mahuang and Gui Zhi in harmonizing Ying and Wei (Nutritive and Defensive Qi), rather than dispersing them. Modern practitioners often hesitate to use Mahuang and Gui Zhi regardless of severity or whether warming covers are applied to induce sweat, simply because they have not investigated the intent of Zhang Zhongjing. Mahuang and Gui Zhi are agents for harmonizing Ying and Wei; if used in heavy doses with warming covers to induce sweat, they become agents for dispersing Ying and Wei; if used in light doses without warming covers to avoid inducing sweat, they become formulas for harmonizing Ying and Wei.

[Collected Explanations]
Wu Renju stated: To disperse exterior pathogens, Shigao is always used in combination. This is likely because Shigao is cold in nature, and cold can overcome heat; its taste is thin, and thinness can reach the exterior. It is unlike Huang Qin (Scutellariae Radix, 黄芩) and Lian Qiao (or Huang Lian, Coptidis Rhizoma, 黄连), which are cold in nature but bitter and thick in taste, unable to ascend to the surface.

In Shang Han (Cold Damage), presenting with no significant fever, dry mouth and thirst, irritability, and slight aversion to cold on the back, Bai Hu Jia Ren Shen Tang (White Tiger Plus Ginseng Decoction) is the primary treatment.

[Note]
When Shang Han presents with no significant body heat, absence of irritability and thirst, a normal mouth sensation, and aversion to cold on the back, Fu Zi Tang (Aconite Decoction) is used, which belongs to Shao Yin disease. In the current case, Shang Han presents with no significant body heat, indicating that heat is gradually moving from the exterior to the interior. Dry mouth, thirst, and irritability indicate that heat has entered the Yang Ming channel. Although there is a symptom of slight aversion to cold on the back, which seems like Shao Yin, Shao Yin syndrome is characterized by a normal mouth sensation; here, dry mouth and thirst indicate the mouth is not normal. The aversion to cold on the back is not due to Yang deficiency, but rather Yang Ming internal heat steaming upwards to the back, causing sweating and skin laxity, leading to slight aversion to cold. Bai Hu Tang (White Tiger Decoction) is primarily used to directly target the Yang Ming channel and clear heat significantly; Ren Shen (Ginseng Radix, 人参) is added to protect the lax skin/muscle layer.

[Collected Notes]
Yu Chang stated: The differentiation in this clause is very detailed. The pulse must be slippery and floating. There is no significant heat over the whole body, nor aversion to cold, only slight aversion to cold in the back area, indicating that exterior pathogens are about to resolve. The patient's dry mouth, thirst, and irritability indicate that interior heat is already blazing. One must not delay; immediate clearing and resolving are necessary. If delayed, deep heat and exhausted fluids will make treatment futile.

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In Shang Han, if the exterior syndrome is unresolved and there is water qi under the heart (epigastrium), presenting with dry retching, fever, and cough, or alternatively thirst, diarrhea, choking sensation, dysuria with lower abdominal fullness, or wheezing, Xiao Qing Long Tang (Minor Blue Dragon Decoction) is the primary treatment.

[Note]
"Exterior syndrome unresolved" refers to the persistence of symptoms such as floating-tight pulse, headache, body pain, fever, absence of sweat, and aversion to cold. "Water qi under the heart" refers to dry retching and cough. However, water pathology manifests in various ways; hence, the text mentions: alternatively thirst, diarrhea, choking, dysuria with lower abdominal fullness, or wheezing. These are all signs of water qi retention. Therefore, Xiao Qing Long Tang should be used with appropriate modifications for each case. The Classics state: The San Jiao (Triple Burner) is the official of irrigation, through which water passages flow. The Bladder is the official of the capital, storing fluids, which are excreted only when Qi transformation occurs. When the Tai Yang (Greater Yang) channel receives pathogens, if there is no water qi, the disease remains in the channels; if there is water qi, the disease invades the fu organs. Invading the fu organs causes the Qi transformation of the Bladder to fail and the water qi of the San Jiao to lose its pathway. Retention in the Upper Jiao causes cough, wheezing, or choking; retention in the Middle Jiao causes thirst, dry retching, or fullness; retention in the Lower Jiao causes dysuria, lower abdominal fullness, or diarrhea. Wherever water flows, pathology may occur. Xiao Qing Long Tang releases exterior excess of Tai Yang externally and dissipates cold-fluid retention in the San Jiao internally. It is a potent diaphoretic agent, sharing fame with Da Qing Long Tang. One treats heat-agitation due to Tai Yang exterior excess, while the other treats cold-fluid retention due to Tai Yang exterior excess.

[Collected Notes]
Cheng Zhi stated: This clarifies the method of dispelling cold and purging fluid retention when the exterior syndrome of Shang Han is unresolved and water accumulates under the heart.
Wang Hu stated: According to "Ming Li Lun" (Theory Clarification), Qing Long Tang indeed treats diseases caused by both wind and cold. In Shang Han with unresolved exterior, Mahuang can induce sweating; in Feng Biao (Wind Strike) with unresolved exterior, Gui Zhi can disperse. Precisely because the exterior is unresolved and compounded by water qi under the heart, neither of the two previous decoctions can disperse the pathogen; only Xiao Qing Long Tang can eliminate both exterior and interior pathogens.