Source 出处 · Shanghan Lun · Doses are not given here: they are set by a licensed practitioner for the individual.
Safety. Dang Gui Si Ni Tang answers one sentence in the classical text: hands and feet that are cold, and a pulse that is thin and on the point of vanishing. The design is easy to follow. Dang gui and shao yao nourish and hold the blood; gui zhi warms the channels and moves; xi xin is acrid and reaches into the deep cold; gan cao and da zao support the middle; tong cao opens the passage. What the formula is arguing is that cold in the extremities is not always a problem of too little fire - it can be a problem of too little of what carries the warmth outward. That is a genuinely different reading from the one behind si ni tang, which answers the same cold hands with aconite and dried ginger, and the two formulas sitting side by side is the clearest short lesson the tradition offers on how it reasons about cold.
Cold hands and feet are also, today, a sign with a differential, and the differential is the part that matters. Raynaud phenomenon, peripheral arterial disease, hypothyroidism, anaemia, diabetic peripheral neuropathy, orthostatic hypotension, sepsis, hypothermia and frostbite, systemic sclerosis with its calcinosis and sclerodactyly, and ordinary anxiety with overbreathing all produce it. Most are chronic and most are assessed electively. One is not: a limb that becomes cold, painful, pale and pulseless over hours is acute limb ischaemia, and it is a vascular surgical emergency measured in hours, not a pattern to be warmed. A sudden cold, painful, pulseless limb is the version of this symptom that has a clock attached, and this entry says so because the classical wording - hands and feet cold - does not distinguish between the two.
The safety notes here are mostly about names. Xi xin is Asarum, a member of the birthwort family, and it contains aristolochic acid: the same compound behind irreversible interstitial nephropathy and the urothelial cancers that follow it, with cumulative exposure and no established safe threshold. This is why the pharmacopoeia restricts the herb to root and rhizome - the aerial parts carry more of it - and why a preparation that does not say which part it used is not acceptable. Tong cao is the second naming problem: it may be tetrapanax pith, it may be the stem of Stachyurus or Helwingia, and in the classical text it is a subject of genuine disagreement that this entry will not adjudicate. What it must never be is guan mu tong, the aristolochia species whose medicinal standard was cancelled in two thousand and three. Gui zhi contains coumarin, for which European food-safety authorities have set a tolerable daily intake on the basis of liver toxicity in susceptible people, and cassia cinnamon carries considerably more coumarin than the Ceylon species. Shao yao in modern practice is usually white peony root with the bark removed, which is not interchangeable with red peony. Finally, sustained liquorice produces a syndrome that looks like aldosterone excess - raised blood pressure, low potassium, oedema - and interacts with diuretics, digoxin and antihypertensives.
当归四逆汤回应的是古典文献里的一句话：手足厥寒，脉细欲绝者。方义不难循：当归与芍药养血和血，桂枝温经通脉，细辛辛散入深寒，甘草与大枣补中，通草通其道。此方所论的是：四肢之寒未必都是火不足，也可能是把温运送出去的东西不足。这与四逆汤对同一「手足寒」的回答 —— 用附子与干姜 —— 是两条不同的读法，两方并列正是传统如何推演「寒」这一件事最简短的一课。
而手足冰冷在今天同样是一个带鉴别的体征，鉴别才是要紧的部分。雷诺现象、周围动脉疾病、甲状腺功能减退、贫血、糖尿病周围神经病变、体位性低血压、败血症、低温与冻伤、伴钙质沉着与指端硬化的系统性硬化症，以及单纯的焦虑伴过度换气，都可致手足冰冷。其中多数是慢性的、可以择期评估；有一种不是：在数小时内转为冷、痛、苍白、无脉的肢体是急性肢体缺血，属血管外科急症，以小时计，不是需要温的证。突然发作的冷、痛、无脉，是这个症状里挂了钟的那一种，本条目之所以写明，是因为古典原文「手足厥寒」这一句并不区分这两者。
安全提示几乎全在名称上。细辛是细辛属（Asarum）植物，属马兜铃科，含马兜铃酸 —— 正是导致不可逆转的肾小管间质肾病及其后尿路上皮癌的同一类成分，具累积暴露且无确定安全阈值。这正是药典把该药限定为根及根茎的理由（地上部分含量更高），也是不写用哪个部位的制剂不可接受的理由。通草是第二个命名问题：它可能是通脱木的茎髓，也可能是旌节花或青荚叶的茎髓，古典原文究竟指何物历来有争议，本条目不作裁决。但绝不可是关木通 —— 那味在二〇〇三年被取消药用标准的马兜铃属药材。桂枝含香豆素，欧洲食品安全机构基于易感人群的肝毒性已为其设定每日可耐受摄入量，而中国肉桂（cassia）所含香豆素显著高于锡兰肉桂。芍药在现代习用中多为去外皮的白芍，与赤芍不可互代。最后，甘草持续服用会产生类似醛固酮增多的表现 —— 血压升高、低钾、水肿，并与利尿剂、地高辛及降压药存在相互作用。