杜仲，味辛、甘，气平温，降也，阳也，无毒。入肾经，补中强志，益肾添精，尤治腰
痛不
能屈伸者神效，亦能治足、阴囊湿痒，止小水梦遗。此物可以为君，而又善为臣使，但嫌过
燥，与熟地同用，则燥湿相宜，自然无火动之忧也。
或问肾恶燥，而杜仲性燥，何以入肾以健腰？吾子加熟地尤宜，然亦似熟地之滋肾，终
非杜
仲之益肾矣。曰∶补肾原不必熟地，余用熟地者，不过取其相得益彰也。夫肾虽恶燥，而湿
气侵之，腰即重着而不可俯仰，是肾又未尝不恶湿也。杜仲性燥，燥肾中之邪水，而非烁肾
中之真水也。去熟地而肾中之燥不相妨，用熟地而肾中之湿亦无碍，盖杜仲自能补肾，而非
借重于熟地之助也。
或问杜仲非燥药也，而吾子谓是燥药，何据而云然乎？曰∶论杜仲之有丝，其非燥药也。
然
而杜仲之燥，正有有丝之不肯断。夫太刚则折，大柔则不肯折矣。杜仲之丝，经火炒则断，
其中之柔软为何如，而独谓其性燥者，别有义也。杜仲不经
火则湿，经火则燥。不断之丝，非火炒至无丝，则不可为未非受火气迫急而为燥乎。肾
恶燥。而
以燥投燥，遽入往往动火，我所以教人与熟地同用也。至于肾经中湿，不特宜同熟地并施，
且宜生用为妙，并不可火炒。盖肾既有湿，得熟地则增润，反牵制杜仲。一加火，则失其本
性，但补而不攻，而湿邪反不得遽散。夫杜仲不炒则湿，何反宜于治湿。盖杜仲燥中有湿，
湿非水气之谓也。邪湿得真水而化，生用，正存其真气耳。
或问杜仲补肾，仲景公何故不采入八味丸中？不知杜仲补肾中之火，而有动肾气，动则
桂、
附不安于肾宫，恐有飞越之虞，故用桂、附，而不用杜仲。然则固不可用乎，肾中有湿气，
正宜加用于八味丸中，取其动而能散湿也，又不可拘执不用而尽弃之耳。
或问杜仲补肾，世人竟以破故纸佐之，毋乃太燥乎？杜仲得破故纸，而其功始大，古人
嫌其
太燥，益胡桃仁润之，有鱼水之喻。其实，杜仲得破故纸，正不必胡桃仁之润也。盖破故纸
温补命门之火，而杜仲则滋益肾中之水，水火有既济之美，又何必胡桃仁之润哉。虽杜仲得
胡桃仁之相助，亦无碍其益肾之功。然而，杜仲实无借于胡桃仁也。或云胡桃仁滋破故纸之
燥也。夫破故纸用之于他药之中，未见用胡桃仁之助，何独入于杜仲之中而加胡桃仁也。谓
非因杜仲而入之，吾不信也。
杜仲，味道辛、甘，气性平温，具有沉降作用，属阳，无毒。进入肾经，能补益中气、增强意志，滋养肾脏、增添精液，尤其对腰痛导致无法屈伸的症状有显著疗效，也能治疗足部及阴囊潮湿瘙痒，止住小便频多和梦中遗精。此药可以作为主药（君药），又善于作为辅佐药（臣使药），只是担心其过于燥热，若与熟地黄同用，则燥湿相互调和，自然没有引发内火的忧虑。
有人问：肾脏厌恶燥邪，而杜仲性燥，为何能入肾以强健腰膝？我建议您加入熟地黄更为适宜，但这似乎变成了熟地黄在滋补肾阴，终究不是杜仲在直接益肾了。回答说：补肾原本不一定非要用熟地黄，我使用熟地黄，不过是取其相得益彰的效果罢了。肾脏虽然厌恶燥邪，但如果湿气侵犯肾脏，腰部就会沉重且不能俯仰，这说明肾脏也未尝不厌恶湿气。杜仲性燥，是燥化肾中的邪水，而不是灼烧肾中的真阴之水。去掉熟地黄，肾中的燥邪并不相妨；使用熟地黄，肾中的湿气也无妨碍，因为杜仲本身就能补肾，并非依赖熟地黄的帮助。
有人问：杜仲并不是燥药，而您却说是燥药，有什么依据呢？回答说：论杜仲的丝状物，它确实不是燥药。然而杜仲的“燥”，正体现在那丝状物不肯断裂的特性上。太刚硬的东西容易折断，太柔软的东西则不肯折断。杜仲的丝，经过火炒就会断裂，其中的柔软程度如何，之所以单独说它性燥，另有深意。杜仲不经火炒则偏湿，经火炒则变燥。那不断裂的丝，如果不是火炒到没有丝的程度，难道不是受火气逼迫而变得燥吗？肾脏厌恶燥邪，如果用燥药投之，往往会引起动火，所以我教导人们要与熟地黄同用。至于肾经中有湿气，不仅适宜与熟地黄并用，而且生用最为妙处，不可火炒。因为肾中既有湿气，得到熟地黄则增加滋润，反而牵制杜仲。一旦加火炒，就失去了本性，只能补而不能攻散，湿气反而不能迅速消散。杜仲不炒则湿，为什么反而适合治湿呢？这是因为杜仲燥中寓湿，这里的湿并非指水气。邪湿得到真水才能化解，生用正是保存其真气罢了。
有人问：杜仲补肾，张仲景为何不将其采入八味丸中？不知道杜仲补的是肾中之火，但会扰动肾气，肾气动荡则桂枝、附子不能在肾宫安守，恐怕有飞越的危险，所以用桂枝、附子，而不用杜仲。那么杜仲就一定不可用了吗？如果肾中有湿气，正适宜在八味丸中加入杜仲，取其能动而能散湿的作用，不可拘泥固执不用而完全抛弃它。
有人问：杜仲补肾，世人竟用破故纸（补骨脂）来辅助它，岂不是太燥了吗？杜仲得到破故纸，功效才宏大，古人嫌弃它太燥，便加入胡桃仁来润燥，这好比鱼水相得的关系。其实，杜仲得到破故纸，正不必非要胡桃仁来润燥。因为破故纸温补命门之火，而杜仲则滋益肾中之水，水火既济之美，又何必需要胡桃仁的滋润呢？虽然杜仲得到胡桃仁的帮助，也不妨碍其益肾的功效。然而，杜仲实际上并不需要借助胡桃仁。有人说胡桃仁是用来滋润破故纸的燥性的。破故纸用在其他药物中，未见需要用胡桃仁帮助，为何唯独放入杜仲中就加上胡桃仁呢？如果不是因为杜仲才加入的，我不相信。
Eucommia ulmoides (Duzhong), with pungent and sweet flavors, has a neutral and warm nature, possesses descending properties, belongs to Yang, and is non-toxic. It enters the Kidney Meridian, tonifies the middle jiao, strengthens willpower, benefits the Kidney, and replenishes essence. It is particularly effective for lower back pain accompanied by inability to flex or extend, and can also treat dampness and itching of the feet and scrotum, as well as stop frequent urination and nocturnal emission. This herb can serve as the sovereign drug (Jun Yao) and is also adept at serving as an attendant drug (Chen Shi). However, it is somewhat drying; when used together with Prepared Rehmannia Root (Shudi), the dryness and dampness are balanced, naturally avoiding the worry of stirring up internal heat.
One might ask: The Kidney dislikes dryness, yet Eucommia is drying in nature; how can it enter the Kidney to strengthen the waist? I suggest adding Prepared Rehmannia Root, which is more suitable, but this seems to make Prepared Rehmannia Root nourish the Kidney, rather than Eucommia benefiting the Kidney directly. The response is: Tonifying the Kidney does not necessarily require Prepared Rehmannia Root. I use Prepared Rehmannia Root merely to achieve a synergistic effect. Although the Kidney dislikes dryness, if dampness invades the Kidney, the waist becomes heavy and unable to bend or straighten, indicating that the Kidney also dislikes dampness. Eucommia's drying nature dries out pathogenic water in the Kidney, rather than scorching the true Yin fluids of the Kidney. Removing Prepared Rehmannia Root does not hinder the treatment of dryness in the Kidney, and using Prepared Rehmannia Root does not obstruct dampness in the Kidney, because Eucommia can tonify the Kidney on its own, without relying on the assistance of Prepared Rehmannia Root.
One might ask: Eucommia is not a drying herb, yet you claim it is. What is your basis? I reply: Regarding the fibrous strands of Eucommia, it is indeed not a drying herb. However, the "dryness" of Eucommia lies precisely in the resistance of these strands to breaking. Things that are too rigid break easily, while things that are too soft do not break. The fibers of Eucommia break when stir-fried with fire; given their inherent softness, why single out its nature as drying? There is another meaning. Eucommia is damp when unprocessed and becomes dry after stir-frying. Those unbroken fibers, unless stir-fried until they no longer exist, must have been forced into dryness by the heat. Since the Kidney dislikes dryness, administering a drying herb often stirs up fire, which is why I teach people to use it with Prepared Rehmannia Root. As for dampness in the Kidney Meridian, it is not only suitable to be used with Prepared Rehmannia Root, but raw usage is best, and stir-frying should be avoided. If the Kidney already has dampness, adding Prepared Rehmannia Root increases moisture, which counteracts Eucommia. Once stir-fried, it loses its original nature, becoming purely tonifying without dispelling, so the damp pathogen cannot be scattered quickly. Why is Eucommia, which is damp when unroasted, suitable for treating dampness? This is because Eucommia contains both dryness and dampness; here, "damp" does not refer to water qi. Pathogenic dampness transforms upon contact with true water; using it raw preserves its true qi.
One might ask: Eucommia tonifies the Kidney, yet why did Zhang Zhongjing not include it in the Eight-Ingredient Pill with Rehmannia (Ba Wei Wan)? It is unknown whether Eucommia tonifies the fire in the Kidney but disturbs Kidney Qi. If Kidney Qi is disturbed, Cinnamon Twig (Guizhi) and Aconite Root (Fuzi) cannot remain stable in the Kidney palace, fearing the risk of floating away. Therefore, Cinnamon Twig and Aconite Root are used instead of Eucommia. Does this mean Eucommia is absolutely unusable? If there is dampness in the Kidney, it is appropriate to add Eucommia to the Eight-Ingredient Pill, taking advantage of its ability to move and scatter dampness. One should not stubbornly refuse to use it and discard it entirely.
One might ask: Eucommia tonifies the Kidney, yet people commonly use Psoralea Fruit (Pogu Zhi / Buguzhi) to assist it. Is this not too drying? Eucommia achieves great efficacy when combined with Psoralea Fruit. Ancients disliked its excessive dryness and added Walnut Kernel (Hetao Ren) to moisten it, likening the relationship to fish and water. In reality, when Eucommia is combined with Psoralea Fruit, there is no need for Walnut Kernel to moisten. Psoralea Fruit warms and tonifies the Fire of the Gate of Vitality (Ming Men), while Eucommia nourishes and benefits the Water of the Kidney. This creates the beauty of Water and Fire interacting harmoniously (Ji Ji); why is Walnut Kernel's moistening necessary? Although Eucommia benefits from the assistance of Walnut Kernel, it does not hinder its Kidney-tonifying function. However, Eucommia essentially does not rely on Walnut Kernel. Some say Walnut Kernel moistens the dryness of Psoralea Fruit. When Psoralea Fruit is used with other herbs, there is no seen need for Walnut Kernel's assistance. Why is Walnut Kernel added specifically when used with Eucommia? If it were not because of Eucommia, I would not believe it.