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The primary indication of Rehmanniae Radix (Dihuang, 地黄) is the treatment of blood disorders and water diseases.

Textual Research
In the syndrome pattern for Ba Wei Wan (Eight-Ingredient Pill with Rehmannia Root), it states: "insensitivity in the lower abdomen." It also states: "difficulty in urination."
In the above formula, the dosage of Rehmanniae Radix is eight liang.
In the syndrome pattern for Xiong Gui Jiao Ai Tang (Cnidium, Angelica, Donkey-Hide Gelatin, and Moutan Decoction), it states: "persistent dripping bleeding." It also states: "lower body bleeding."
In the above formula, the dosage of Rehmanniae Radix is six liang.
In the syndrome pattern for San Wu Huang Qin Tang (Three-Ingredient Scutellaria Decoction), it states: "after childbirth on a straw mat, exposure to wind, causing severe irritability and heat in the four limbs."
In the above formula, the dosage of Rehmanniae Radix is four liang.
From a comprehensive review of these three formulas, their primary indications are blood disorders and water diseases, excluding other conditions.

Cross-Reference
Xiong Gui Jiao Ai Tang, San Wu Huang Qin Tang, and Ba Wei Wan all use Rehmanniae Radix as the Jun Yao (Monarch Drug). Two of them address blood disorders, while one addresses difficulty in urination.
In the composition of Jiao Ai Tang, besides Rehmanniae Radix, there are Ecliptae Herba (Ejiao, 阿胶), Angelicae Sinensis Radix (Danggui, 当归), and Chuanxiong Rhizoma (Chuanxiong, 川芎), all of which are drugs for treating blood disorders. In San Wu Huang Qin Tang, if Rehmanniae Radix is removed, the remaining ingredients have no efficacy in treating blood disorders. From this perspective, it is evident that ancient practitioners used Rehmanniae Radix to treat both blood disorders and water diseases simultaneously; this has been verified. Furthermore, it clarifies that the therapeutic method does not strictly distinguish between blood and water.

Correction of Misconceptions
Water and blood are fundamentally of the same category. The only distinction is that red fluid is called blood, and white fluid is called water. I once read in the Huangdi Neijing (Yellow Emperor's Inner Canon): "Sweat is the surplus of blood." One might ask: If sweat is the surplus of blood, why is it white? The answer given is: "The Lung governs the skin and hair; the color of the Lung is white, therefore sweat is white." This theory is based on Yin-Yang and the Five Elements, but it harms the practice of clinical medicine. Has the way of clinical medicine perished? The cause of this situation is truly lamentable! The reason sweat is white and blood is red is unknowable. When a blade touches the skin, even if the wound is shallow, blood will definitely flow. During hot summer weather, or when wearing thick clothing and bedding, sweat will definitely flow. Both sweat and blood exit through the skin and hair; some become sweat, others become blood. Therefore, regarding the reason for their different colors, we should acknowledge the unknown, set it aside without discussion, and focus solely on treating the pathogenic factors at their location. This is the true way of clinical medicine. Later physicians who regarded Ba Wei Wan as a kidney-tonifying agent were extremely erroneous. Zhang Zhongjing stated: "For foot qi ascending, causing insensitivity in the lower abdomen, Ba Wei Wan is indicated"; he also said: "For difficulty in urination"; he also said: "For bladder dysfunction (Zhuan Bao), healing occurs upon promoting urination"; he also said: "For shortness of breath due to mild fluid retention, it should be eliminated via urination." All these applications highlight its function in promoting urination. The book says: "Learning from ancient teachings leads to acquisition." Alas! Only by learning from ancient classics can one truly master the efficacy of herbs.

Product Examination
Rehmanniae Radix: Produced everywhere in our country. The production in Hezhou is the most abundant and is indistinguishable from that produced in Han territory (mainland China); it should be solid and substantial. Zang Qi stated: "Ben Jing (Shennong Ben Cao Jing) does not mention raw, dried, or steamed forms. Bie Lu (Mingyi Bie Lu) states: Raw Dihuang refers to freshly dug material; Li Shizhen stated: Cooked Dihuang is produced by later generations through repeated steaming and drying, and various materia medica texts refer to dried Dihuang as cooked Dihuang. However, currently, pharmacies in our country refer to dried Dihuang as raw Dihuang, which is incorrect. 'Dried' refers to the process of drying, such as Gan Jiang (Dried Ginger); 'Raw' refers to freshness, such as Sheng Jiang (Fresh Ginger). Thus, when ancients mentioned Sheng Dihuang, they necessarily referred to its juice, which makes logical sense. How could a dried herb contain juice? The materials used by Zhang Zhongjing were limited to two types: Sheng Di (Raw) and Gan Di (Dried). The so-called Shu Di (Cooked) is a later invention and should not be used."