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The flavor is sweet, the nature is slightly cold, and it is non-toxic. It is suitable to be thoroughly decocted with half wine and half water. After drying the liquid together in the sun, steaming and sun-drying should be repeated nine times as the limit. Copper and iron vessels must be avoided. If incorporated into formulas for Spleen deficiency, it should be stir-fried until dry; for patients with phlegm, it should be stir-fried with ginger juice.

Shu Di Huang (Prepared Rehmanniae Radix) greatly tonifies blood deficiency, doubles the nourishment of Kidney Water, fills the bone marrow to consolidate True Yin, specifically tonifies the Yuan Qi within the Kidneys, and concurrently treats the meridians that store blood. It addresses injuries from falls and fractures involving severed tendons, as well as the five labors and seven injuries, blood bi syndromes, and internal injuries of the five zang organs. It repairs broken vessels, unblocks blood vessels, benefits qi and strength, sharpens hearing and vision, darkens hair, clears deficient heat, moistens dryness, replenishes essence and blood, and regulates menstruation. For post-typhoid fever cases where the shins and abdomen are most painful, it shows remarkable efficacy; for acute abdominal pain after recent childbirth, it produces immediate results. As a substance among turbid substances, it strengthens the bone marrow. Internal injury diseases often affect the liver tendons and kidney bones. Shu Di Huang specifically nourishes the Liver and Kidneys, and is essential for internal injuries affecting tendons, bones, marrow, and kidneys.

Indications (Integrated with Smallpox Theory): Nourishes Kidney Water, tonifies blood to benefit True Yin, can calm the soul and spirit, and treats smallpox lesions lacking pus due to blood deficiency. It is suitable for those with blood deficiency during or after smallpox, as it tonifies the Yuan Qi of the Kidneys, which is the source generated by Tian Yi (Heavenly One/Primordial Origin). However, its nature is stagnant and does not circulate freely. If used by those with Spleen deficiency, it must be soaked in wine and stir-fried.

Commentary: Shu Di Huang is an essential herb for tonifying the Kidneys and a superior agent for nourishing yin. In Liu Wei Di Huang Wan (Six-Ingredient Pill with Rehmannia), it serves as the sovereign drug, reflecting the principle of Tian Yi as the root. In Si Wu Tang (Four Substance Decoction), it serves as the sovereign drug, reflecting the principle that Yi and Gui share the same source (Liver and Kidney share the same origin). Only through nine steamings and nine sun-dryings is it properly prepared. It is common to see people mistakenly consider simply boiled Rehmannia as Shu Di Huang. This is incorrect because Shu Di Huang inherits the pure yin nature of the North; without the refining process of sunlight (gentle heating) and fierce fire, it cannot become "cooked." Therefore, in Gu Ben Gao (Solidify the Root Paste), although subjected to prolonged simmering, raw and prepared Rehmannia must be used in equal halves. This illustrates the point. If one fails to understand this and uses cooked Sheng Di Huang (Raw Rehmanniae Radix) as Shu Di Huang in Rehmannia pills, the cold-natured property remains unremoved. The medicinal properties for the Heart meridian and Kidney meridian become separated: using cold-natured drugs for the Heart meridian as sovereigns, while using warm-natured drugs for the Kidney meridian as ministers and assistants. This not only yields no benefit but also introduces coldness, damaging both True Yang and Stomach Qi. Those with deficient heat may temporarily endure it, but those with deficient cold will immediately suffer severe illness. The Yin aspect suffers silently, unnoticed. What a pity!