伤寒阴阳易之为病。其人身体重。少气。少腹里急。或引阴中拘挛。热上冲胸。头重不欲举。眼中生花。膝
胫拘急者。烧 散主之。
旧注云。大病新瘥。气血未复。余热未尽。强合阴阳而得病者。名曰易。男子病新瘥。
未平复而妇人与之交。妇人得病。名曰阳易。妇人病新瘥。未平复而男子与之交。男子得病。名曰阴易。以愚
意推之。盖以二气 。其构精之时。乃化醇之候也。二气不杂。两精融一。故能化生。所谓二五之精。妙合而凝也。
然男病易之于女。女病易之于男。其受病之人。并非气血未复者。实为注家之误。但男女一交之后。自然元气空虚。
余邪错杂于精气之中。走入精隧。溢入经络。乘其交后虚隙之中。入而浸淫于脏腑筋骨。脉络 穴之间。则正气
因邪而益虚。邪气因虚而益盛。故有此阴盛阳
衰之诸证也。邪入阴经。身体必重。真阳亏损。三焦不运。宗气不行。所以少气。邪从阴窍而溜入少阴厥阴。
故少腹里急。若里急之甚。或引阴中拘挛。皆阴邪之所致也。阴邪在下而虚阳上走。故热上冲胸。头重不欲举。
眼中生花。下焦虚冷。所以膝胫拘急也。此真所谓阴阳之患。故以烧 散主之。
烧 散方
上取妇人中 近隐处。剪烧灰。以水和服方寸匕。日三服。小便即利。阴头微肿则愈。妇人病。取男子
裆烧灰。
或谓阴阳易之证。近于变幻。故用烧 之法。亦近于厌禳。曰非也。有至理存焉。男女之交媾。易所谓二
气感应以相与也。以未净之邪。随交合之情。精神魂魄。无不动摇。翕然而感。感而遂通。混入于少阴之里。
故以近隐处之 裆。引出其阴中之邪。所谓物从其类。同气相求之义也。但此方当为导引之药。其余当随其脉症
之阴阳寒热。治之可也。如王海藏之脉在厥阴。当以当归四逆汤。下烧 散。在少阴。当以通脉四逆汤下烧 散。
在太阴。当以理中丸同下烧 散。所用之药。各随其经而效自速也。不然。则有阴肿或缩。少腹绞痛。手足蜷。
脉绝而死者。其后人之青竹茹汤。及 鼠粪汤。恐未必能合原文中之诸阴症也。大凡易病。必有如前诸证者方是。
如无此见证。不过传染之证耳。
伤寒病后出现的“阴阳易”这种病症。患者身体沉重，气短乏力，小腹部拘急疼痛，或者牵引阴部痉挛收缩，热气上冲至胸部，头部沉重不想抬起，眼前发黑视物模糊，膝盖和小腿拘挛僵硬。这种情况应当用烧裈散来主治。
旧注解说：大病刚刚痊愈，气血尚未恢复，余热还没有清除干净，此时强行进行房事而导致的疾病，称为“易”。如果男子大病初愈，身体尚未平复就与女子发生性关系，女子因此得病，称为“阳易”；如果女子大病初愈，身体尚未平复就与男子发生性关系，男子因此得病，称为“阴易”。依我推究，大概是因为男女交合精气之时，正是化生醇厚的时机。两气不混杂，两精融合为一，所以能化生生命，这就是所说的“二五之精，妙合而凝”。然而，男病传给女，女病传给男，受病的人，并不是因为气血未恢复才生病的，这实际上是旧注的错误之处。只是男女一次性交之后，元气自然空虚，残余的病邪错杂在精气之中，侵入精道，溢入经络，趁着性交后身体的虚弱空隙，进入并浸淫于脏腑筋骨、脉络穴位之间，那么正气因为邪气而更加虚弱，邪气因为正气虚而更加强盛，所以出现这些阴盛阳衰的各种症状。邪气侵入阴经，身体必然沉重；真阳亏损，三焦运化失职，宗气运行不畅，所以气短乏力。邪气从阴窍溜入少阴经和厥阴经，所以小腹部拘急疼痛。如果拘急疼痛严重，甚至牵引阴部痉挛收缩，这都是阴邪所致。阴邪在下，虚阳上浮，所以热气上冲胸部，头部沉重不想抬起，眼前发黑视物模糊。下焦虚冷，所以膝盖和小腿拘挛僵硬。这确实可以说是阴阳失调引起的病患，所以用烧裈散来主治。
烧裈散方剂组成及用法：
取妇人中衣（贴身内裤）靠近隐处部分，剪下来烧成灰，用水调和，服用一方寸匕的量。每天服用三次，小便通畅后，阴头轻微肿胀，病就好了。如果是女子患病，则取男子的裆部衣物烧成灰服用。
有人议论说，阴阳易的症状变幻莫测，所以使用烧裈的方法，也接近于巫术厌禳。我说不是这样，其中蕴含着深刻的道理。男女之间的交媾，正如《易经》所说，是二气感应而相结合。将未净尽的病邪，随着交合的情欲，使精神魂魄无不摇动，迅速感应，感通之后，混入少阴经的内部。所以用靠近隐处的裈裆，引出潜伏在阴分中的邪气，这就是所谓“物从其类，同气相求”的道理。但是这个方子应当作为引导药使用，其余的治疗应当根据患者的脉象和症状的阴阳寒热性质来决定。比如王海藏认为，如果脉象显示病在厥阴经，应当用当归四逆汤送服烧裈散；病在少阴经，应当用通脉四逆汤送服烧裈散；病在太阴经，应当用理中丸同烧裈散一起服用。所用的药物，各自跟随所属经脉而疗效自然迅速。否则，就会出现阴囊肿大或内缩，小腹部绞痛，手足蜷曲，脉搏消失而死亡的情况。后世使用的青竹茹汤及鼠粪汤，恐怕未必能符合原文中所述的这些阴证。大凡阴阳易的病，必须有前面提到的那些症状才是真的阴阳易。如果没有这些见证，不过是普通的传染病罢了。
The disease known as "Yin-Yang Exchange" (Yin Yang Yi, a condition transmitted between partners after recovery from severe febrile disease) in post-shanghan syndrome. The patient experiences heaviness of the body, shortness of breath (Shao Qi), urgency and tightness in the lower abdomen, or spasms drawing into the genitalia, heat rushing up to the chest, heaviness of the head making it difficult to lift, blurred vision or seeing spots before the eyes, and stiffness/spasms of the knees and shins. This condition should be primarily treated with Shao Kun San (Burnt Underwear Powder).
Old annotations state: After a major illness has just healed, if sexual intercourse is forced while the vital qi and blood have not yet recovered and residual pathogenic heat remains, resulting in illness, it is named "Exchange". If a man, newly recovered from illness and not yet fully restored, engages in intercourse with a woman, and she contracts the disease, it is called "Yang Exchange" (Yang Yi). If a woman, newly recovered from illness and not yet fully restored, engages in intercourse with a man, and he contracts the disease, it is called "Yin Exchange" (Yin Yi). In my opinion, this reasoning is flawed. The essence of conception lies in the harmonious blending of two qi during intercourse. However, the person contracting the disease through exchange is not suffering because their qi and blood were un-recovered; rather, they are healthy but become infected. The error lies in the old annotations. It is only that after sexual intercourse, the primordial qi naturally becomes deficient. Residual pathogens mix within the essence and qi, entering the spermatic channels, overflowing into the meridians, and taking advantage of the void left by intercourse to infiltrate the zang-fu organs, sinews, bones, and acupoints. Consequently, righteous qi becomes more deficient due to the pathogen, and the pathogen becomes more vigorous due to deficiency, leading to these symptoms of yin excess and yang decline. Pathogens entering the yin meridians cause body heaviness; loss of true yang and failure of the San Jiao (Triple Burner) to transport, along with stagnation of Zong Qi (Gathering Qi), result in shortness of breath. Pathogens slipping from the yin orifices into the Shaoyin (Lesser Yin) and Jueyin (Terminal Yin) meridians cause lower abdominal urgency. Severe urgency may draw into genital spasms, all caused by yin pathogens. With yin pathogens below and deficient yang floating upward, heat rushes to the chest, the head feels heavy and hard to lift, and vision blurs. Cold in the lower jiao causes stiffness of the knees and shins. This is truly a disorder of yin and yang, hence treated with Shao Kun San.
Formula: Shao Kun San (Burnt Underwear Powder)
Take the part of the woman's undergarment closest to the genitals, cut it off, burn it to ash, mix with water, and administer one cun-bi (a measure roughly equivalent to a tablespoon) per dose. Take three times daily. When urination becomes smooth and the glans penis swells slightly, the patient will recover. For women suffering from this, take the man's undergarment裆 (crotch area), burn it to ash, and administer similarly.
Some argue that the symptoms of Yin-Yang Exchange are变幻 (capricious/changing), so using burnt underwear seems akin to witchcraft or apotropaic magic. I say this is incorrect; there is profound logic herein. Sexual intercourse is the mutual interaction of two qi, as stated in the I Ching. Unpurified pathogens, driven by sexual desire, shake the spirit and soul, inducing rapid resonance and penetration into the interior of the Shaoyin meridian. Thus, using the underwear from the genital region draws out the pathogens hidden in the yin, following the principle of "things follow their kind, and similar qi seeks each other." However, this formula should serve only as a guiding agent. Subsequent treatment must be based on the yin-yang and cold-heat nature of the pulse and symptoms. For instance, Wang Haizang suggested: if the pulse indicates involvement of the Jueyin meridian, administer Shao Kun San with Danggui Sini Tang (Cinnamon Twig and Angelica Decoction); if involving Shaoyin, use Tongmai Sini Tang (Pulse-Reviving Decoction); if involving Taiyin, use Lizhong Wan (Regulate the Middle Pill). Medications should correspond to the affected meridian for rapid efficacy. Otherwise, complications such as scrotal swelling or retraction, severe lower abdominal colic, curling of hands and feet, and absent pulse leading to death may occur. Later formulas like Qingzhuru Tang (Green Bamboo Shoot Decoction) and Shufen Sui Tang (Rat Feces Decoction) likely do not align with the yin-syndromes described in the original text. Generally, for Yin-Yang Exchange to be diagnosed, the aforementioned symptoms must be present. Without these specific manifestations, it is merely an infectious disease.