Natrii Sulfas — the refined crystalline form of the mineral mirabilite (hydrated sodium sulfate), purified and recrystallised before it becomes the drug. It is a mineral salt, not a plant; the same crystal, dehydrated, is the related material 玄明粉 (Natrii Sulfas Exsiccatus).
Nature and flavour. Salty, bitter, very cold. Stomach and Large Intestine systems.
The coldest and saltiest member of the purge category, and the only one that is a mineral. The sources classify it by two verbs: it softens what is hard, and it drains what has accumulated downward. It is recorded for the pattern the texts call heat accumulation with dry stool and abdominal fullness, and externally for hardness, swelling, sore breast and painful haemorrhoids — where the traditional application is the salt's drawing, cooling action rather than any internal effect. It is the standing partner of rhubarb in the classical purgation combinations: the bitter purge and the saline purge are understood as acting together. Its action is osmotic — the salt holds water in the bowel. That much is physiology and it is well described. The classical indications attached to it are not established by controlled evidence.
性味归经。咸、苦，大寒。归胃、大肠经。
泻下药里最寒、最咸的一味，也是唯一一味矿物。文献以两个动词归它：软坚，与泻下。记载用于古籍所称热结便秘、腹满硬痛，以及外用治硬结、肿痛、乳痈、痔疮肿痛——外用的原理是盐的吸水与清凉，而不是内服的效力。它是古典泻下组合里大黄的固定搭档：苦寒攻下与咸寒软坚被视为相辅相成。其作用机制是渗透压——盐把水留在肠腔。这一部分是生理学，描述得清楚；而附于其上的古典主治，并没有对照证据的支持。
这是一味盐类泻药，风险全由此而来。一次泻下所需的用盐量，对高血压、心衰、肾病、肝硬化腹水、水肿以及限盐饮食者，恰好是最要紧的一群人。水泻可致脱水与电解质紊乱，反复使用会造成不用泻药就排不出的肠道。它不是日常补品，不是减重手段，也不宜长期服。腹痛原因未明、疑似肠梗阻、阑尾炎、炎症性肠病者禁用；孕期、哺乳期、儿童、体弱或已脱水者同样禁用。两个混淆值得点名，因为两者都危险：硝石（硝酸钾）与芒硝是不同矿物、不同性质，不可互换；硫酸镁（泻盐）又是另一种盐。只有药典级材料才可用作药物，工业级硫酸钠既不是食品也不是药品。请放在儿童拿不到的地方。