通、行水、泻下焦血分湿热。
大辛苦寒。太阳经药。（膀胱。）能行十二经。通腠理。利九窍。泻下焦血分湿热。为疗
风水
之要药。（十剂曰∶通可去滞、通草防己之属是也、古之通草、即今之木通是、徐之才亦以
行水者为通、与燥剂无以别矣、木通甘淡、泻气分湿热、防己苦寒、泻血分湿热。）主治膀
胱火邪。热气诸痫。（降气下痰。）湿疟香港脚。（足伤寒湿为香港脚、寒湿郁而为热、湿则
肿、热则痛、防己为主药、湿加苡仁苍术木瓜木通、热加知己、风加羌活萆 、痰加竹沥南
星、
四物、大便闭加桃仁红花、小便闭加牛膝泽泻、痛连臂加桂枝威灵仙
、痛连胁加龙胆草、又有足跟痛者、属肾虚、不与香港脚同论。）水肿风肿。痈肿恶疮。性险
而健。阴虚及湿热在上焦气分者禁用。（东垣云∶防己大苦大寒、泻血中湿热、亦瞑眩之药
也、服之使人身心烦乱、饮食减少、唯湿热壅遏、及香港脚病、凡下焦湿热、致二阴不通者、
非此不效、若虚人用防己、其害有三、谷食己亏、复泄大便、重亡其血、一也；渴在上焦气
分、而防己泻下焦血分、二也；伤寒邪传肺经气分、湿热而小便黄赤、禁用血药、三也。）
出汉中。根大而虚。通心有花纹。色黄。名汉防己。黑点黄腥木强者。名木防己。不佳。（
藏器曰∶治风用木防己、治水用汉防己。）酒洗。恶细辛。畏萆 、女菀、咸卤。（防己煎汁
服、能解雄黄毒。）
（防己的功效为）通利、行水、泻除下焦血分的湿热。
其性味大辛、大苦、寒。属于太阳经的药物。（对应膀胱经。）能通行于十二经脉，疏通腠理，通利九窍，泻除下焦血分的湿热，是治疗风水病的重要药物。（《十剂》中说：通法可以去除滞涩，通草、防己这类药物就是代表。古代的通草，就是现在的木通。徐之才也将行水药归为“通”类，这与燥湿药没有区别了。木通味甘淡，泻气分湿热；防己味苦寒，泻血分湿热。）主治膀胱火邪，热气引起的各种痫证。（具有降气下痰的作用。）湿疟疾，香港脚。（足部感受寒湿称为香港脚，寒湿郁结化为热，湿气重则肿胀，热盛则疼痛，防己为主药。若湿气重加薏苡仁、苍术、木瓜、木通；热盛加知母；风邪重加羌活、萆薢；有痰加竹沥、天南星；四物汤调理；大便闭塞加桃仁、红花；小便闭塞加牛膝、泽泻；疼痛牵连手臂加桂枝、威灵仙；疼痛牵连胁肋加龙胆草；又有足跟痛者，属于肾虚，不与香港脚同论。）水肿，风肿，痈肿恶疮。药性险峻而功效强健。阴虚及湿热在上焦气分者禁用。（李东垣说：防己大苦大寒，泻血中湿热，也是令人产生瞑眩反应的药物。服用后使人身心烦乱，饮食减少。只有湿热壅遏，以及香港脚病，凡是下焦湿热导致前后二阴不通的，非此药不效。如果虚人使用防己，其危害有三：一是谷食已经亏损，又泄大便，再次耗伤血液，这是一害；二是渴感在上焦气分，而防己泻下焦血分，这是二害；三是伤寒邪气传入肺经气分，湿热导致小便黄赤，禁用血分药，这是三害。）产于汉中。根部大而空虚，中心有通心花纹，颜色黄，名为汉防己。有黑点、黄色、腥味重、木质坚硬的，名为木防己，品质不佳。（陈藏器说：治风用木防己，治水用汉防己。）用酒洗过。畏忌细辛。害怕萆薢、女菀、咸卤环境。（防己煎汁服用，能解雄黄毒。）
(Its functions are) promoting urination, moving water, and draining damp-heat from the blood level of the lower jiao.
It is greatly acrid, bitter, and cold. It is a herb for the Taiyang channel. (Corresponds to the Bladder.) It can circulate through the twelve channels, open the interstices, benefit the nine orifices, and drain damp-heat from the blood level of the lower jiao. It is an essential herb for treating Fengshui edema. (The "Ten Categories" states: The category of 'Tong' (Unblocking) removes stagnation; herbs like Tongcao and Fangji belong to this. Ancient Tongcao is equivalent to modern Mutong. Xu Zhicai also classified water-moving herbs as 'Tong', which was indistinguishable from drying herbs. Mutong is sweet and bland, draining damp-heat from the qi level; Fangji is bitter and cold, draining damp-heat from the blood level.) It primarily treats Fire Evil in the Bladder and various epilepsy syndromes caused by hot qi. (It descends qi and resolves phlegm.) It treats Damp Malaria and Hongjiaojiao (Gouty Arthritis). (Hongjiaojiao refers to foot conditions caused by cold-dampness invading the feet; when cold-dampness stagnates and transforms into heat, dampness causes swelling, and heat causes pain. Fangji is the principal herb. For heavy dampness, add Coix Seed, Atractylodes Macrocephala, Quince, and Mutong; for heavy heat, add Anemarrhena; for wind, add Notopterygium and Dioscorea Hypoglauca; for phlegm, add Bamboo Sap and Arisaema Cum Bile; use Siwu Decoction; for constipation, add Peach Kernel and Carthamus Flower; for urinary retention, add Achyranthes Root and Alisma Rhizome; for pain radiating to the arms, add Cinnamon Twig and Syringa Vulgaris Root Bark; for pain radiating to the hypochondrium, add Gentian Root; heel pain belongs to Kidney Deficiency and is not discussed here with Hongjiaojiao.) It treats edema, wind-swelling, carbuncles, and malignant sores. Its nature is perilous yet vigorous. It is contraindicated in Yin Deficiency and cases where damp-heat resides in the qi level of the upper jiao. (Li Dongyuan stated: Fangji is greatly bitter and cold, draining damp-heat from the blood, and is also a drug that induces Mingxuan reactions (severe adverse reactions leading to recovery). After ingestion, it causes mental restlessness and loss of appetite. It is effective only when damp-heat is obstructed, particularly in Hongjiaojiao, where damp-heat in the lower jiao causes obstruction of the two yin passages. If used in deficient persons, there are three harms: first, if grain and food intake is already depleted, further purging the bowels doubly destroys blood; second, thirst located in the qi level of the upper jiao is treated by Fangji draining the blood level of the lower jiao, which is mismatched; third, when Shaoyang pathogenic factors transmit to the Lung channel's qi level, causing yellow-red urine due to damp-heat, blood-level drugs are contraindicated.) It originates from Hanzhong. The root is large and pithy, with a central star-like pattern, and is yellow; this is named Han Fangji. Those with black spots, yellowish color, strong fishy odor, and hard wood are named Mu Fangji, but they are of inferior quality. (Chen Cangqi said: Use Mu Fangji for treating wind, and Han Fangji for treating water.) It should be washed with wine. It is incompatible with Ephedrae Herba (Mahuang). It fears Dioscoreae Hypoglaucae Rhizoma (Bixie), Polygala Tenuifolia Radix (Nuwan), and saline environments. (Decoction of Fangji can detoxify Realgar.)