Semen Lini — the dried ripe seed of Linum usitatissimum L. (family Linaceae); the same plant that gives linen fibre and linseed oil.
Nature and flavour. Sweet, neutral. Lung, Liver and Large Intestine systems.
An oily seed living in two traditions at once. In Chinese sources it is a moist laxative for dryness-type constipation, and it is also recorded for the patterns the texts describe as wind-damp with numbness and stiffness, and for dry, scaling skin. In the West the same seed is a food ingredient, a source of alpha-linolenic acid and of lignans, and a bulk laxative in its own right. The mucilage in the husk swells with water and the fibre adds bulk — that is the ordinary explanation of the laxative reputation, and it is a better explanation than the classical one. None of the traditional Chinese indications beyond the bowel are established by evidence. Note that this is not 火麻仁: hemp seed is a different plant and a different drug, and the two oily seeds are regularly confused by name.
性味归经。甘，平。归肺、肝、大肠经。
一味同时活在两个传统里的油料种子。中文文献里它是治燥结便秘的润下药，也记载用于古籍所称风湿麻木、筋脉拘挛与皮肤干燥脱屑。在西方，同一种子是食品原料、α-亚麻酸与木酚素的来源，本身也是容积性泻剂。种皮的黏质吸水膨胀，纤维增加粪便体积——这是对「通便」声誉的通常解释，也比古典解释更站得住。除通便之外的中医主治，都没有证据支持。请注意它不是火麻仁：大麻仁是另一味药，两种油料种子常被名字混淆。
最重要的一条是容积性泻剂的规则：这枚种子会膨胀。若干吞或饮水不足，可能造成梗阻。文献中确有食道与肠道梗阻的报告，风险人群是食道或肠道狭窄、有吞咽困难，或已有严重便秘粪嵌塞者——对他们，这是选错的药。除此以外，这枚种子是食物，被广泛食用，告诫都是通常的那些。它含氰苷（其中有亚麻苦苷），所以大量生食碾碎的种子与面包上撒一勺是两回事；加热可降低，日常烹饪用量不会成为问题。它会延缓胃内其他东西的吸收，因此应与正在服用的其他药物错开时间，而非同服。它可能降低血糖，使用降糖药的人需要知道这一点。种子含有具雌激素样作用的木酚素，这在激素相关疾病或孕期是否有影响尚未明确——理由已经足够：那种情况下应当避免，而不是去试。整粒种子常常原样排出；研碎的种子与整粒表现不同，而亚麻油又是另一回事。