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Physicians in the North often use Da Yuan Yin (Reach the Origin Decoction) to treat Wen Bing (Warm Disease), which is a major error. Wu Shi stated that during the Xin Si year of the Chongzhen reign, epidemic qi spread widely, with particularly many infections in Shandong and Zhejiang provinces along both north and south routes; thus, he wrote the book *Wen Yi Lun* (Treatise on Plague). The first prescription recorded was Da Yuan Yin, intended as a formula for the initial stage of plague, not originally designed for treating Wen Bing. "Yi" (Plague/Epidemic) refers to the malevolent qi of heaven and earth, containing toxic bacteria, spreading throughout the environment like servitude, hence named "Yi". Since plagues mostly present with heat syndromes, they are called "Wen Yi" (Warm Plague; those presenting with cold syndromes are called "Han Yi"); "Wen" means "Warm". Therefore, this formula focuses on expelling abnormal qi. In contrast, Wen Bing arises from contracting the warm qi of the season, or from external cold previously contracted lying latent in the Mo Yuan (Membrane Source), transforming into heat over time and erupting when the season arrives; it contains no toxic bacteria and is not contagious. Treatment should solely aim at clearing and resolving heat, after which the disease can be cured. If differentiation is not precise, and a patient suffering from Wen Bing is mistakenly given Da Yuan Yin, the breaking-down power of Bin Lang (Areca Seed) in the formula can guide warm qi inward, while the acrid-warm dispersing nature of Hou Po (Magnolia Bark) and Cao Guo (Tsaoko Fruit) severely consumes yin fluids and assists heat, which is especially unsuitable for patients with Wen Bing (patients with Wen Bing often have yin deficiency and particularly must avoid herbs that consume yin). Although the formula contains Zhi Mu (Anemarrhena Rhizome), Bai Shao (White Peony Root), and Huang Qin (Scutellaria Root), each at one qian, their cooling power is very weak. Consequently, there has never been a case where using this formula cured Wen Bing. Moreover, not only does it fail to cure, but taking it at the onset of illness can suddenly transform the condition into a critical state. This is not my baseless speculation, but based on actual observations.

When I first studied medicine, I once observed an elderly woman over sixty years old who, due to excessive grief, had accumulated chronic fatigue disease. In mid-spring, she contracted Wen Bing. The physician prescribed Da Yuan Yin, modifying the dosage of Cao Guo to one qian, stating that sweating would lead to recovery. However, after ingestion, no sweat appeared, and symptoms seemed to worsen. The physician then doubled the dose of Cao Guo, claiming sweating was inevitable. Indeed, after taking the modified dose, profuse sweating occurred on the head and face, severe panting ensued, and within moments, the patient suffered from collapse (sudden loss of yang qi). Some suspected the failure of this case lay in increasing the dose of Cao Guo in Da Yuan Yin; if the original dosage of five fen were used, even several doses should not have led to sweating-induced collapse. I replied: Cao Guo has a very potent nature; even five fen is not a small amount. I once treated a patient with spleen-deficiency diarrhea unresponsive to medication. Recalling that Si Shen Wan (Four Spirits Pill) is effective for dawn diarrhea and contains Rou Dou Kou (Nutmeg), which materia medica states strengthens the spleen and binds the intestines, I administered half a qian of powdered Rou Dou Kou with decoctions of spleen-strengthening herbs. Shortly after, the patient felt inner instability, and all six pulses disappeared, reappearing only after half an hour. I suddenly realized that the patient's weak constitution could not withstand the acrid-dispersing power of Rou Dou Kou. Cao Guo shares similar properties with Rou Dou Kou, but its acrid-dispersing force is even more intense. Although only five fen was used in the formula, combined with Bin Lang and Hou Po, its potent strength is certainly significant. From this, we can see how dangerous it is to use Da Yuan Yin lightly!