Source 出处 · Shanghan Lun · Doses are not given here: they are set by a licensed practitioner for the individual.
Safety. The three Cheng Qi formulas of the Shanghan Lun are the standard illustration of how much of Chinese formula study is really the study of small differences. All three contain rhubarb. Da Cheng Qi Tang adds mirabilite, and its indications are described with the four words pi, man, zao, shi - blockage, fullness, dryness and hardness - which is about as compressed a clinical picture as the literature ever produces. Xiao Cheng Qi Tang has no mirabilite and no liquorice; it has magnolia and bitter orange, so it moves qi more than it softens what is dry. Tiao Wei Cheng Qi Tang keeps the mirabilite and drops the qi-moving pair, and is aimed at dryness and heat rather than at distension. Remove one ingredient and the target of the formula moves. That is the teaching point, and it is a real one.
There is a second and more uncomfortable passage attached to this formula, and it deserves to be quoted rather than quietly dropped. The text describes a patient who has not opened his bowels for six or seven days and says that if one suspects dry stool is present, there is a way to find out: give a small amount of Xiao Cheng Qi Tang, and if wind is passed after the decoction goes down, dry stool is indeed there and the attack may proceed; if no wind is passed, it is only the head of the stool that is hard, and one must not attack. In other words the purge is being used as a diagnostic instrument. It is a genuinely clever piece of reasoning from a tradition with no imaging and no laboratory, and this entry will not pretend it is not there.
It is also not a practice to reproduce, and the reason is not prudishness. Today, a person who has not opened his bowels for a week is either obstructed or not, and the two cases have opposite treatments: an obstructed bowel is made worse by stimulant laxatives, and if it is distended and friable the outcome can be perforation. Ultrasound and plain film answer in minutes what the decoction answered in hours. Alongside that, the modern differential for what the old text called bound interior is not one thing: severe constipation with abdominal distension covers faecal impaction, sigmoid volvulus, obstructing colorectal tumour, paralytic ileus, opioid-induced constipation, hypothyroidism, hypercalcaemia, and medication effects from anticholinergics, calcium channel blockers, iron and several psychiatric drugs.
On the materials. Rhubarb is an anthraquinone stimulant laxative; used repeatedly and over time it produces dependence, fluid and potassium loss, and melanosis coli, and the potassium loss matters specifically for anyone taking digoxin or a potassium-wasting diuretic. Magnolia bark is the hou po of many formulas and is read today through honokiol and magnolol; wild stands of Magnolia officinalis have been heavily harvested and the species is subject to protection measures, so provenance is not a decorative detail here. Immature bitter orange is the third, and it is the one that should be described plainly rather than reassuringly: it contains p-synephrine, and extracts of bitter orange came under close regulatory attention in the years after ephedra alkaloids were withdrawn from the supplement market in the United States, with cardiovascular adverse events reported for stimulant combinations. Note also that zhi shi and zhi ke are not the same item - one is the immature fruit, the other the nearly mature one - and that traditional practice distinguishes them.
The summary this knowledge base will stand behind: the three Cheng Qi formulas are the clearest worked example in the literature of what a substitution does, the diagnostic use recorded in the source belongs to its own century, and a week without a stool is a clinical question, not a formula selection.
《伤寒论》的三首承气汤是「中医方剂学究竟在研究什么」的标准例证：它们其实都在研究微小的差别。三首都用大黄。大承气汤加了芒硝，它的主治用「痞、满、燥、实」四个字来写，大概是这份文献里最压缩的一幅临床画面。小承气汤没有芒硝也没有甘草，而有厚朴与枳实，所以它行气的成分多于软燥的成分。调胃承气汤留住芒硝、去掉行气的一对，对准的是燥与热而不是胀。抽掉一味，方的靶子就移动了。这是教学要点，而且是一个真的要点。
另有一段更不好处理的记载也挂在此方上，值得引出来而不是悄悄跳过去。原文描写一位六七日不大便的病人，说若怀疑有燥屎，有个办法可以探明：给少量小承气汤，汤入腹中转矢气者，是有燥屎，可以攻之；若不转矢气，只是初头硬，便不可攻。换句话说，这里是把泻下当成诊断工具来用的。它确实是一个没有影像、没有化验的传统里相当聪明的推演，本条不打算假装它不存在。
它同样不是今天应当复现的做法，而理由并不是拘谨。今天一个七天未解大便的人，属于肠梗阻或者不属于，而这两种情形的处理正好相反：梗阻的肠管会被刺激性泻药加重，若它已经胀得发脆，结局可以是穿孔。超声与腹部平片用几分钟就能回答那剂汤药要用几个小时回答的问题。与此同时，旧文本所谓「里实」今天的鉴别也不是一件事：严重便秘伴腹胀覆盖粪便嵌塞、乙状结肠扭转、梗阻性结直肠肿瘤、麻痹性肠梗阻、阿片类所致便秘、甲状腺功能减退、高钙血症，以及抗胆碱药、钙通道阻滞剂、铁剂与若干精神类药物的药源性作用。
单味药：大黄属蒽醌类刺激性泻药，反复久服会带出依赖、水与钾的丢失和结肠黑变病，而失钾对正在服用地高辛或排钾利尿剂的人尤其要紧。厚朴是许多方里都出现的那一味，今天主要通过和厚朴酚、厚朴酚来读它；野生厚朴（Magnolia officinalis）被采伐得很重，这个物种受到相应的保护措施，所以药材来源在这里不是可有可无的装饰。第三味枳实是应当被如实描述而不是被安抚的那一味：它含对辛弗林；在麻黄碱类生物碱退出美国补充剂市场之后的那些年里，枳实属苦橙提取物受到密切的监管关注，而兴奋剂组合的心血管不良事件有报告。另需说明，枳实与枳壳不是同一味——前者是幼果，后者是近成熟的果实——传统是区分的。
本站愿意为之负责的概括是这样的：三首承气汤是文献里最清楚的一个「换一味会怎样」的范例；原书里那段诊断用法属于它自己的世纪；而一周不解大便是一个临床问题，不是一次选方。