Catechu (a dried aqueous extract; in Chinese practice chiefly Acacia catechu (L. f.) Willd., with the related gambir from Uncaria gambir treated as a distinct article)
Nature and flavour. Bitter and astringent, slightly cold or cool depending on the source. Lung and Heart systems in the common account.
Not a plant part but an extract: the heartwood is boiled down and the decoction dried into a dark, brittle mass — in effect a concentrated tannin. Classical texts describe it as clearing the Lung and transforming phlegm, and externally as drying dampness, astringing and engendering flesh, and it is recorded for weeping sores and slow-healing ulcers, for eczema, for mouth and throat ulcers as a powder or rinse, and for bleeding from the gums. Internally it is recorded for phlegm-heat cough and for diarrhoea and dysentery. The same material has had a much wider career outside medicine: catechu has long been a tanning and dyeing agent, and it is one of the traditional ingredients of betel quid (paan) across South and Southeast Asia. That last point matters and is worth stating plainly — the quid as chewed, areca nut with slaked lime, is classified as carcinogenic to humans, and chewing it is associated with oral and oesophageal cancer and with oral submucous fibrosis. Those associations belong to the chewing practice and to areca nut; it is not established that catechu on its own accounts for them.
性味归经。苦、涩，微寒（部分本草作凉）。归肺、心经（通行说法）。
它不是植物的一部分而是提取物：心材水煎、浓缩、干燥成深色脆块，实质上是一份浓缩鞣质。古籍记载清肺化痰，外用燥湿、收涩、生肌，用于渗液不收的疮疡与久不敛口的溃疡、湿疹、口舌生疮与咽喉溃烂（作散剂或含漱），以及齿龈出血。内服记载用于痰热咳嗽与泄泻痢疾。同一物质在医药之外还有更广的履历：儿茶历来是鞣革与染色的材料，也是南亚与东南亚蒌叶咀嚼块（paan）的传统配料之一。最后这一点重要，须直说 —— 咀嚼块的实际形态（槟榔加熟石灰）被列为对人类致癌，咀嚼它与口腔癌、食管癌以及口腔黏膜下纤维化相关。这些关联属于咀嚼行为本身与槟榔；儿茶单独是否构成该风险，并未被证实。
一味高鞣质材料，这一点就是安全问题的全部。鞣质收涩，内服则刺激胃：恶心、便秘与腹部不适是常见结果；早年鞣酸曾用于烧伤治疗，后因肝损伤而被放弃 —— 这段临床经历正是内服用途逐渐收窄而非扩大的原因。内服不属于自用范畴，此处所列内服用途属于传统记载而非已确立的证据。外用燥涩：不宜大面积、用于破损皮肤或长期反复使用，并须避开眼部。孕期与哺乳期及儿童避免使用。两点实际提醒：以此名流通的货可能来自不止一种植物，非正规渠道所买未必是文本所写的那一味；正在扩散、渗液或久不收口的疮面需要的是明确诊断，而不是一味收涩药。