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Original text from *Shanghan Lun* (Treatise on Cold Damage Diseases): In Shaoyin disease (a syndrome caused by pathogenic invasion of the Shaoyin meridians), at the initial stage, if there is fever and a deep pulse, Mahuang Fuzi Xixin Tang (Ephedra, Aconite, and Asarum Decoction) should be administered.

This occurs when external cold pathogens penetrate directly from the Taiyang meridian (Foot-Taiyang Bladder Meridian) into the Shaoyin meridian (Foot-Shaoyin Kidney Meridian), constituting a combined disease of Taiyang and Shaoyin. Since it is a combined disease of Shaoyin and Taiyang, the Shaoyin has already been injured by cold. Although there may be fever on the exterior, this is not robust heat (high fever) but rather fever accompanying aversion to cold. Therefore, the pulse is not floating but deep; the pulse of Shaoyin is naturally fine and minute, which is close to a deep pulse. Thus, Fuzi (Prepared Aconite Root) is used to resolve interior cold, Mahuang (Ephedra Herb) is used to resolve exterior cold, and Xixin (Asarum Root) with its pungent, warm, aromatic, and penetrating nature is added as an assistant. Xixin can both assist Fuzi in resolving interior cold and assist Mahuang in resolving exterior cold, allowing the cold that penetrated from the Taiyang to be expelled through sweating via the Taiyang. This demonstrates the exquisite efficacy of Mahuang Fuzi Xixin Tang.

[Mahuang Fuzi Xixin Tang Formula] Mahuang (Ephedrae Herba) 2 liang (removed joints), Xixin (Asari Radix) 2 liang, Fuzi (Aconiti Lateralis Radix Praeparata) 1 piece (processed, skin removed, broken into eight pieces).

These three ingredients are boiled in one dou (approx. 10 liters) of water. First, boil Mahuang until the volume decreases by two sheng (approx. 2 liters), remove the foam on the surface, then add the remaining herbs. Boil until three sheng (approx. 3 liters) remain, strain the dregs, and take one sheng (approx. 1 liter) warm, three times daily.

The Xixin in the formula is 2 liang, which converts to 6 qian in modern measurements. Divided into three doses, each dose contains 2 qian. Later generations held the view that "Xixin dosage should not exceed 1 qian," a notion explicitly refuted by Zhang Yinan. A dosage of 2 qian is not inadvisable, but to alleviate patient anxiety, using 1 qian can also be effective. Generally, for conditions suitable for inducing sweating, the pulse is floating; however, in this specific case, the pulse is deep, necessitating the use of Xixin to assist in inducing sweat. The reason why 1 qian is considered effective is that Xinyin possesses potent properties, so even 1 qian is a substantial amount.

If this formula is applied to early-stage Shaoyin disease where there is only aversion to cold without fever, it can also be used. I once treated a young man during summer. At noon, he consumed watermelon excessively. Due to insomnia the previous night, he fell asleep deeply by the window after eating. A sudden east wind blew, causing the weather to turn cold abruptly, waking him up. Before waking, he had experienced nocturnal emission in his dream. Upon waking, he felt cold and trembling all over, with dull abdominal pain that intensified shortly thereafter. I was summoned urgently. His pulse was minute and faint, almost imperceptible. I prescribed: Mahuang (Ephedrae Herba) 2 qian, Wufuzi (Prepared Aconite Root) 3 qian, Xixin (Asari Radix) 1 qian, Shudihuang (Rehmanniae Radix Preparata) 1 liang, Shengshanyao (Dioscoreae Rhizoma) 5 qian, Jingyurou (Corni Fructus) 5 qian, Ganjiang (Zingiberis Rhizoma) 3 qian, Gongdingxiang (Caryophylli Flos) 10 grains. These were decocted together and taken as a soup. After taking the medicine, he was kept warm under covers, resulting in slight sweating throughout the body, and both the trembling and abdominal pain resolved. Several additional herbs were added to Mahuang Fuzi Xixin Tang because of the sudden deficiency of Shaoyin and the accompanying abdominal pain.

Patient Li XX contracted Shaoyin cold damage in summer and was cured using Mahuang Fuzi Xixin Tang supplemented with Shengshanyao (Dioscoreae Rhizoma) and Dushudi (Rehmanniae Radix Preparata).