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Gansui (Kansui Radix) promotes bowel movement and drains water-dampness from the meridians and collaterals.

It is bitter, cold, and toxic. It can drain water-dampness from the Kidney meridian and the body's deep channels (referring to the deep pathways for fluid metabolism), reaching directly to the sites where water qi condenses. Its primary function is to attack and resolve water accumulation, making it a supreme medicine for purging water (used in Zhang Zhongjing's Da Xianxiong Tang).

It treats twelve types of water diseases and severe abdominal distension and swelling (known as Shuigu [Water Gu]). Yu Jiayan stated: "The Kidney is the sea of food and drink and the source of the five zang and six fu organs. If the Spleen fails to scatter the water essence from the Stomach to the Lung, disease arises in the middle; if the Lung fails to regulate the water passages from the Stomach to the Bladder, disease arises in the upper jiao; if the Kidney fails to control the gate of the Stomach and timely regulate excretion, disease arises in the lower jiao, leading to overflowing accumulated water without cessation. The Kidney is the gate of the Stomach. The anterior orifice benefits water, while the posterior orifice benefits grain." Wang Haogu stated: "Water metabolism is governed by the Spleen, Lung, and Kidney meridians. Water pathology can involve parts of the five zang, six fu, and twelve meridians, affecting the head and face above, limbs in the middle, waist and legs below, skin externally, muscles deeply, and bones internally. Pulse diagnoses vary between cun and chi positions and floating and sinking qualities. One should not lightly use purgative medicines; one must know which meridian and zang-fu are affected before prescribing. Note: Edema may result from phlegm encasing food stagnation and blood stasis, causing clear yang to fail to ascend and turbid yin to fail to descend, constituting an excess syndrome. It may also result from mutual generation of damp-heat and obstruction of channels, also an excess syndrome. Or it may result from dry-heat impact and constipation, also an excess syndrome. Conversely, it may result from taking cold-cooling herbs that damage digestion and cause deficiency of Middle Qi, or from exhaustion of healthy qi after severe illness, or from inability of the Spleen to control water due to urinary retention and chaotic water flow, constituting a deficiency syndrome. These should be treated differently. However, the root cause often lies in deficiency of Middle Qi. Zhu Danxi said: 'In treating water diseases, priority should be given to strengthening the Spleen. When the Spleen is solid and Qi moves, water flows naturally.' Ren Shen (Ginseng Radix) and Fu Ling (Poria) should be used as sovereign drugs, modified according to accompanying syndromes. If only diuretics are used, it often leads to untreatable conditions." It also treats Jiajian masses and hernias, retained fluids and old food, and epilepsy caused by phlegm confusing the mind. It is contraindicated in deficiency cases.

The bark is reddish, the flesh is white, and the root resembles a string of pearls. Those that are heavy and solid are superior. For use, wrap in flour and bake until cooked (or soak in Gancao [Licorice Root] and Qicai juice for three days; when the liquid turns black like ink and then clears, wrap in flour and bake again). Gouti (Melon Stalk) serves as an assistant. It is incompatible with Yuanzhi (Polygalae Radix). It is chemically antagonistic to Gancao (Glycyrrhizae Radix et Rhizoma) (Zhang Zhongjing used it together with Gancao to treat retained fluid under the heart, utilizing their antagonistic interaction to achieve therapeutic effect. In some cases treating edema and sores, applying Gansui powder to the swollen area while administering concentrated Gancao decoction orally causes immediate reduction of swelling, demonstrating such strong reciprocal induction between the two herbs).