《伤寒论》原文∶少阴病，始得之，反发热脉沉者，麻黄附子细辛汤主之。
此外感之寒凉，由太阳直透少阴，乃太阳与少阴合病也。为少阴与太阳合病，是以少阴已为寒凉所伤，而
外表纵有发热之时，然此非外表之壮热，乃恶寒中之发热耳。是以其脉不浮而沉，盖少阴之脉微细，微细原
近于沉也。故用附子以解里寒，用麻黄以解外寒，而复佐以辛温香窜之细辛，既能助附子以解里
寒，更能助麻黄以解外寒，俾其自太阳透入之寒，仍由太阳作汗而解，此麻黄附子细辛汤之妙用也。
【麻黄附子细辛汤方】麻黄二两去节，细辛二两，附子一枚炮去皮破八片。
上三味，以水一斗，先煮麻黄减二升，去上沫，纳诸药，煮取三升，去滓，温服一升，日三服。
方中细辛二两，折为今之六钱，复三分之一剂中仍有二钱，
而后世对于细辛有服不过钱之说，张隐庵曾明辩其非。二钱非不可用，而欲免病家之疑，用一钱亦可奏效。盖
凡宜发汗之病，其脉皆浮，此独脉沉，而欲发其汗，故宜用细辛辅之，至谓用一钱
亦可奏效者，因细辛之性原甚猛烈，一钱亦不为少矣。
此方若少阴病初得之，但恶寒不发热者，亦可用。曾治一少年，时当夏季，午间恣食西瓜，因夜间失眠，
遂于食余当窗酣睡，值东风骤至，天气忽变寒凉，因而冻醒，其未醒之先，又复梦中遗精，醒后遂觉周身寒凉
抖战，腹中隐隐作疼，须臾觉疼浸加剧。急迎为延医，其脉微细若无，为疏方，用麻黄二钱，乌附子三钱，细
辛一钱，熟地黄一两，生山药、净萸肉各五钱，干姜三钱，公丁香十粒，共煎汤服之，服后温复，周身得微
汗，抖战与腹疼皆愈。此于麻黄附子细辛汤外而复加药数味者，为其少阴暴虚腹中疼痛也。
李××，夏日得少阴伤寒，用麻黄附子细辛汤，加生山药、大熟地二味治愈。
《伤寒论》原文：少阴病（少阴经受邪所致的病证），初起时，反而出现发热、脉象沉的，用麻黄附子细辛汤主治。
这是外感寒凉之邪，由太阳经（足太阳膀胱经）直接传入少阴经（足少阴肾经），属于太阳与少阴合病（两经同时受邪发病）。因为是少阴与太阳合病，所以少阴经已经受到寒凉损伤，虽然体表有时会有发热现象，但这并不是体表的壮热（高热），而是恶寒（怕冷）中的发热。因此其脉象不浮而沉，因为少阴经的脉象本微细，微细原本就接近于沉脉。所以使用附子来解除里寒，使用麻黄来解除表寒，并辅以气味辛温、芳香走窜的细辛，它既能帮助附子解除里寒，又能帮助麻黄解除表寒，使得从太阳经透入体内的寒邪，仍然通过太阳经发汗而解除，这就是麻黄附子细辛汤的精妙之处。
【麻黄附子细辛汤方】麻黄二两（去节），细辛二两，附子一枚（炮制后去皮，破成八片）。
以上三味药，用水一斗，先煮麻黄，水量减少二升，去掉水面上的浮沫，加入其他药物，煮取三升药液，滤去药渣，温服一升，每日服用三次。
方中细辛二两，折算为现在的六钱，再分为三分之一剂，每剂中仍有二钱。而后世有“细辛用量不超过一钱”的说法，张隐庵曾明确辩驳这种观点是错误的。二钱并非不可用，但为了免除病人的疑虑，用一钱也能奏效。大概凡是适宜发汗的病证，其脉象都是浮脉，唯独此证脉象沉，想要发汗，所以适宜用细辛辅助。至于说用一钱也能奏效，是因为细辛的药性原本非常猛烈，一钱的剂量也不算少了。
这个方子如果用于少阴病初起，只有恶寒而不发热的情况，也可以使用。我曾治疗一名少年，正值夏季，中午肆意吃西瓜，因为夜间失眠，便在吃完西瓜后对着窗户熟睡，恰逢东风骤起，天气突然变得寒凉，因而被冻醒。在他未醒之前，又在梦中遗精，醒来后便觉得周身寒冷颤抖，腹中隐隐作痛，片刻之后感觉疼痛逐渐加剧。急忙请我去诊治，他的脉象微细若无，我给他开方，用麻黄二钱，乌附子三钱，细辛一钱，熟地黄一两，生山药、净山茱萸各五钱，干姜三钱，公丁香十粒，共煎汤服用。服药后保暖覆被，周身微微出汗，颤抖和腹痛都痊愈了。在这个麻黄附子细辛汤的基础上又加了这几味药，是因为患者少阴暴虚且伴有腹痛。
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).