寒热往来者。主半表半里。其病属少阳。盖少阳当阴阳出入之枢。邪至其地。与正气相
争。相
争则寒。争胜则热矣。此与恶寒发热有别。恶寒发热者。寒热互见。此则寒时自寒而不见热
。热时自热而不见寒也。又与寒热如疟者有别。寒热如疟者。作止有时。此则寒已而热。热
已而寒。一日三五发。甚者十数套。与疟状有以异也。小柴胡汤专治往来寒热。盖以柴胡治
半表。半夏治半里。黄芩生姜交除寒热。而加以人参甘枣扶正逐邪。盖病至少阳。发汗攻
里。皆所不宜。故以是为和解之剂。乃少阳病之定法也。
小柴胡汤主治往来寒热。人皆知之。此特半表证。而半里证。人多忽焉不讲。所谓半里
者。如
口苦心烦喜呕胸胁满痛之类是也。凡邪从太阳而来。其人仍发热。而少阳半里证已见。虽未
往
来寒热。即当从少阳和解。如经云。伤寒四五日。身热。恶风。颈项强。胁下满。手足温。
而渴者。小柴胡汤主之。又云。伤寒后六七日。发热。微恶寒。支节烦疼。心下支结。外证
未去者。柴胡桂枝汤主之是也。至少阳病。来路自太阳。而其去则入阳明之府。其来自太
阳者。如经云。伤寒五六日已发汗而复下之。胁满微结。小便不利。渴而不呕。往来寒热心
烦者。
此为未解也。柴胡桂枝干姜汤主之是也。其欲入府而犹未入者。如经云。伤寒十余日。热结
在里。复往来寒热者。与大柴胡汤是也。
寒热往来。属半表半里证。然有由表而里者。又有由里而表者。不可不辨也。大凡风寒
之邪
。多自表而里。湿热之邪。多自里而表。风寒法。仲景论中详矣。至湿热之邪。伏于募原。
其起病即见呕渴。胸腹满。不大便诸里证。及其发热。往往热已而寒。寒已而热。此其里证
重于表证。宜察其里证之轻重。使里先和。则表自解。盖病自里而表。少
阳正当往来出入之界。故其始往来寒热。继则热多寒少。再则但热不寒。至昼夜壮热。而谵
妄烦渴毕见。此病之由轻入重也。至于由重出轻。则必使谵妄烦渴诸里证先罢。身热渐和。
其时邪气已退。正气未复。又复相争而为往来寒热。此乃病出入之大机。而前之寒热往来为
病进。后之寒热往来为病退。总视其里证之轻重有无为据也。其有邪气全退。表里俱和。而
仍寒热未去。往来如疟者。此因正气未复。宜调其饮食。和其营卫。自然渐愈。切不可骤与
峻补。恐余邪为恋。反增其害矣。
又伤寒往来寒热。与疟相似而实非。凡疟当未作之时。饮啖如平人。至疟作而始作。此
则默默不欲饮食。兼有口苦心烦喜呕胸胁痛诸里证。以此为辨。
寒热往来（忽冷忽热，交替出现），病位主要在半表半里。该病症归属于少阳经。因为少阳经处于阴阳出入的枢纽位置，邪气到达此处，与人体正气相互抗争。相互抗争时表现为恶寒，正气战胜邪气时则表现为发热。这与单纯的恶寒发热有所不同：恶寒发热是寒感和热感同时存在；而寒热往来则是感到寒冷时只觉寒冷而不发热，感到发热时只觉发热而不发冷。
这又与“寒热如疟”（症状像疟疾一样的寒热交替）有所区别。“寒热如疟”是有发作和停止的时间规律；而寒热往来则是冷过之后发热，热退之后又变冷，一天发作三到五次，严重的甚至十几次，这与疟疾的症状是有区别的。小柴胡汤专门治疗寒热往来。其中柴胡治疗半表之邪，半夏治疗半里之邪，黄芩和生姜共同清除寒热，再加入人参、甘草、大枣扶助正气以驱逐邪气。病至少阳阶段，使用发汗法或攻下法都不适宜，因此用小柴胡汤作为和解少阳的方剂，这是治疗少阳病的固定法则。
人们都知道小柴胡汤主治寒热往来，但这只是针对半表证而言，对于半里证，很多人却忽视不讲。所谓的半里证，包括口苦、心烦、喜欢呕吐、胸胁部胀满疼痛等症状。凡是邪气从太阳经传入，患者仍然发热，且少阳半里证已经显现，即使还没有出现寒热往来，也应当按少阳病进行和解治疗。正如《伤寒论》所说：“伤寒四五日，身体发热，怕风，颈项强直，胁下胀满，手脚温暖，并且口渴的，用小柴胡汤主治。”又说：“伤寒后六七天，发热，轻微怕冷，四肢关节烦痛，胃脘部有支撑结聚感，外部表证未解除的，用柴胡桂枝汤主治。”
至于少阳病，其来路自太阳经，而其去路则可能转入阳明腑。来自太阳经的情况，如《伤寒论》所说：“伤寒五六天，已经发汗却又误用攻下法，导致胁下胀满、轻微结聚，小便不利，口渴但不呕吐，出现寒热往来、心烦的，此为病邪未解，应用柴胡桂枝干姜汤主治。”那些想要转入阳明腑但尚未完全入里的情况，如《伤寒论》所说：“伤寒十多天，热邪结聚在体内，同时又出现寒热往来的，用大柴胡汤主治。”
寒热往来属于半表半里证，然而其中有由表入里的，也有由里出表的，不可不加以辨别。大凡风寒之邪，多从体表向体内传变；湿热之邪，多从体内向体表透发。关于风寒的治法，张仲景在《伤寒论》中已有详细论述。至于湿热之邪，潜伏于募原（膜原，指脏腑之间的间隙），起病时就可见到呕吐、口渴、胸腹胀满、大便不通等里证。等到发热时，往往是热退后才发冷，冷退后又发热。这种情况下，里证重于表证，应当观察里证的轻重，使里气先调和，表证自然解除。因为病势由里出表，少阳正处于往来出入的交界地带，所以起初是寒热往来，接着变为热多寒少，再后来变成只有发热而没有恶寒，直至昼夜壮热，并出现胡言乱语、烦躁、极度口渴等症状，这是病情由轻转重的表现。
至于由重转轻，则必须使胡言乱语、烦躁、口渴等里证先消失，身体发热逐渐平和。此时邪气已退，但正气尚未恢复，正邪再次相争而出现寒热往来。这是疾病出入变化的关键机制，之前的寒热往来代表病情进展，之后的寒热往来代表病情衰退，总之以里证的轻重有无为依据。如果有邪气完全消退，表里俱和，但寒热仍未去除，症状像疟疾一样往来，这是因为正气未复，应当调理饮食，调和营卫，自然会逐渐痊愈。切不可突然给予峻猛的补药，恐怕残留的邪气被留恋住，反而增加危害。
另外，伤寒引起的寒热往来，与疟疾相似但实际上不同。凡是疟疾，在未发作时，饮食起居如同正常人，直到疟疾发作时才出现症状；而伤寒寒热往来者，则默默不想吃东西，兼有口苦、心烦、喜欢呕吐、胸胁痛等里证，以此作为鉴别依据。
Alternating chills and fever (alternating episodes of cold and heat) indicates a pathogenic location at the half-exterior and half-interior level. This pathology belongs to the Shaoyang (Lesser Yang) channel. The Shaoyang serves as the pivot for the entry and exit of Yin and Yang. When pathogens reach this region, they contend with healthy Qi. Contention manifests as chills, while the victory of healthy Qi over pathogens manifests as fever. This differs from simultaneous aversion to cold and fever; in simultaneous aversion to cold and fever, both sensations occur concurrently. In alternating chills and fever, one feels only cold without fever during the chill phase, and only heat without chills during the fever phase.
This also differs from "chills and fever resembling malaria." Malaria-like chills and fever have regular intervals of onset and cessation. Alternating chills and fever involve cycles of cold followed by heat, or heat followed by cold, occurring three to five times a day, or even more than ten times in severe cases, which distinguishes it from malaria. Xiao Chai Hu Tang (Minor Bupleurum Decoction) is specifically indicated for alternating chills and fever. Bupleuri Radix (Chaihu) treats the half-exterior aspect, Pinelliae Rhizoma Preparatum (Banxia) treats the half-interior aspect, Scutellariae Radix (Huangqin) and Zingiberis Recens Rhizoma (Shengjiang) jointly resolve cold and heat, while Ginseng Radix et Rhizoma (Renshen), Glycyrrhizae Radix et Rhizoma (Gancao), and Jujubae Fructus (Dazao) support healthy Qi to expel pathogens. Once the disease reaches the Shaoyang stage, diaphoresis (sweating methods) and purgation are contraindicated. Therefore, Xiao Chai Hu Tang is used as a harmonizing formula, representing the standard treatment method for Shaoyang diseases.
It is widely known that Xiao Chai Hu Tang treats alternating chills and fever, but this primarily addresses the half-exterior syndrome. The half-interior syndrome is often neglected. The half-interior syndrome includes symptoms such as bitter taste in the mouth, irritability, frequent vomiting, and fullness/pain in the chest and hypochondrium. If pathogens transmit from the Taiyang (Greater Yang) channel, and the patient still has fever along with evident Shaoyang half-interior symptoms, even if alternating chills and fever have not yet appeared, treatment should follow the Shaoyang harmonizing principle. As stated in the classic text: "In Shanghan (Cold Damage) after four or five days, if there is body heat, aversion to wind, stiffness of the neck and nape, fullness below the costal margin, warmth of the hands and feet, and thirst, Xiao Chai Hu Tang should be administered." It also states: "After Shanghan, six or seven days later, if there is fever, slight aversion to cold, discomfort and pain in the joints, subcostal fullness/stagnation, and exterior symptoms persist, Chai Hu Gui Zhi Tang (Bupleurum and Cinnamon Twig Decoction) should be administered."
Regarding Shaoyang disease, its origin is from the Taiyang channel, but its progression may enter the Yangming Fu organs. Cases originating from Taiyang include: "After five or six days of Shanghan, if sweating was performed followed by inappropriate purgation, resulting in fullness below the costal margin with slight stagnation, difficulty in urination, thirst without vomiting, and alternating chills and fever with irritability, this indicates unresolved pathogen. Chai Hu Gui Zhi Gan Jiang Tang (Bupleurum, Cinnamon Twig, and Dried Ginger Decoction) should be administered." Cases where the pathogen intends to enter the Fu organs but has not yet done so include: "After more than ten days of Shanghan, if heat is consolidated internally and alternating chills and fever recur, Da Chai Hu Tang (Major Bupleurum Decoction) should be administered."
Alternating chills and fever belong to the half-exterior and half-interior syndrome. However, one must distinguish between transmission from exterior to interior and from interior to exterior. Generally, Wind-Cold pathogens tend to transmit from the exterior to the interior, whereas Damp-Heat pathogens tend to emerge from the interior to the exterior. Zhang Zhongjing detailed the treatment principles for Wind-Cold in his treatises. Regarding Damp-Heat pathogens latent in the Mu Yuan (Membrane Source), the onset often presents with interior symptoms such as vomiting, thirst, abdominal fullness, and constipation. When fever occurs, it typically follows a pattern of heat then cold, or cold then heat. In such cases, interior symptoms outweigh exterior ones. One must assess the severity of interior symptoms; once the interior is harmonized, the exterior will resolve spontaneously. Since the disease moves from interior to exterior, passing through the Shaoyang pivot, the initial stage features alternating chills and fever, progressing to more heat than cold, then to heat without cold, eventually leading to sustained high fever throughout the day and night, accompanied by delirium, restlessness, and extreme thirst. This represents the progression from mild to severe.
Conversely, recovery from severe to mild requires the resolution of interior symptoms like delirium and thirst first, followed by the gradual calming of body heat. At this stage, pathogens have retreated, but healthy Qi has not yet recovered. Residual contention between healthy Qi and lingering pathogens causes alternating chills and fever. This marks the critical mechanism of disease movement: prior alternating chills and fever indicate disease progression, while subsequent occurrences indicate disease regression. The determination relies on the presence and severity of interior symptoms. If pathogens have completely cleared and the interior and exterior are harmonious, yet chills and fever resembling malaria persist, this is due to un-recovered healthy Qi. Treatment should focus on regulating diet and harmonizing Ying (Nutritive) and Wei (Defensive) Qi, leading to natural recovery. Sudden use of strong tonics must be avoided, as it may retain residual pathogens and exacerbate the condition.
Furthermore, alternating chills and fever in Shanghan resemble malaria but are fundamentally different. In malaria, patients eat and drink normally before an attack, with symptoms appearing only during the episode. In contrast, patients with Shanghan alternating chills and fever exhibit apathy, loss of appetite, and accompany their condition with interior symptoms such as bitter taste, irritability, vomiting, and chest/hypochondriac pain. These features serve as the basis for differentiation.