Semen Pruni — the dried ripe kernel of Prunus japonica Thunb. or Prunus humilis Bunge (family Rosaceae); a stone-fruit seed, from the same family as the apricot, the peach and the plum.
Nature and flavour. Acrid, bitter, sweet, neutral. Spleen, Large Intestine and Small Intestine systems.
The gentle branch of the purge category, and the one the sources reach for when the harsh cathartics are unsuitable: constipation in the elderly, after illness, after childbirth, and in anyone the texts describe as depleted. The kernel is oily, and the traditional reading is that the oil moistens a dry bowel rather than forcing it — the difference between 润下 and 攻下 is a real distinction in the literature, not a polite way of saying the same thing. The sources also record it for oedema and abdominal fullness, on the classical reasoning that it moves qi and promotes urination as well as opening the bowel; that reasoning is pre-modern and the claim is not established. The laxative action is usually attributed to the fixed oil in the seed. The kernel also contains amygdalin, and that is the safety problem rather than the mechanism.
性味归经。辛、苦、甘，平。归脾、大肠、小肠经。
泻下药里温和的一支，也是文献在峻下药不宜时转向的那一支：老人、病后、产后，以及古籍所谓虚损之人的便秘。种仁含油，传统的读法是油润燥肠，而不是强行攻下——「润下」与「攻下」在文献里是真实的区分，不是同一件事的客气说法。文献也记载其用于水肿与腹满，理由是它能行气、利水兼通便；这一推理属前现代，该说法未被证实。泻下作用通常归因于种子里的脂肪油。种仁另含苦杏仁苷，那是安全问题，不是作用机制。
先把氰化物那一段说在前面，因为从一味温和润下药里，人们不会想到它。这枚种仁与杏、桃、樱、李的仁一样含苦杏仁苷：碾碎或咀嚼时，苷与同种仁里的酶相遇，就会释放氰化氢。经典的中毒报告来自当作保健食品食用的苦杏仁，而苦杏仁在本传统里是另一味药，传统处理更严格。教训同样适用：种仁不是零食，不宜给儿童，在家里碾碎后一勺一勺吃正是最不该做的事；加热可降低但不能可靠去除氰苷成分。氰化物中毒起于头痛、恶心、头晕与呼吸急促，那是急症，不是睡一觉能过去的事。然后才是普通泻药的告诫：腹痛原因未明或疑似梗阻不用，不作日常用药，久服同样会像其他泻药那样造成依赖与电解质问题。古典主治未被证实。