夫用药之法，贵乎明变。如风会有古今之异，地气有南北之分，天时有寒暑之更，禀赋
有浓薄之别，受病有新旧之差，年寿有老少之殊，居养有贵贱之别。用药之际，勿好奇，勿
执一，勿轻妄，勿迅速。须慎重精祥，圆融活变。不妨沉会以期必妥，药于是乎功成。昔先
贤未有发明，后学因而弗讲，其误世也不既多乎。
夫病有宜补，以泻之之道补之。病有宜泻，以补之之道泻之。病有宜寒剂者，以热剂为
响导之兵。病有宜热剂者，以寒剂为类从之引。病在上者治下，病在下者治上。病同也而药
异，病异也而药同。其义至微，学人最宜深究。
用药之忌，在乎欲速。欲速则寒热温凉行散补泻未免过当。功未获奏，害已随之。药无
次序，如兵无纪律，虽有勇将，适以勇偾事。又如理丝，缓则可清其绪，急则愈坚其结矣。
药有君臣佐使，味有轻重浓薄，人尽知之矣。及其用药也，令人复煎其渣，不知既经煎
沸，则轻且薄者，业已无味。重且浓者，不减初煎。君臣佐使之宜，果安在哉。病浅者犹无
大害，病深者切勿为之。
凡修丸剂，须每种各为细末，以末之轻重合之，则分两方准。不然，易细者一磨无遗，
难碎者三复不尽。卤 若此，何怪其无功哉。
凡药苦者直行而泄，辛者横行而散，酸者束而收敛，咸者止而软坚。独是甘之一味，可
升可降，可浮可沉，可内可外，有和有缓，有补有泻。盖土味作甘，土位居中，而能兼乎五
行也。
凡药之在土者，中半以上为根，其气上行，病在中上焦者用之。中半以下为梢，其气下
行，病在下焦者用之。药之出土者，中半以上为苗，其气味上升。中半以下为身为干，其气
味中守下达咸宜。因其病而酌之，使弗悖乎阴阳也。
凡药在上者，不厌频而少。在下者，不厌顿而多。少服则滋荣于上，多服则峻补于下。
凡病在上者，先食而后药。病在下者，先药而后食。病在四肢者，宜饥食而在昼，病在
骨髓者，宜饱食而在夜。
凡煎药用水，也各有宜。如治湿肿浮胀之疾，而欲使利水道，则取长流水。以流长源远，
其性通达，直引四肢之间也。如治二便不通，及足胫以下风湿，则取急流水。以其湍纵峻
急，其性速下也。如治痰饮郁滞，而欲吐发升散，则取逆流水。以其性逆倒流，洄澜涌决也。
如治中气不足，则取春雨水，有阳道发生之意也。如治下元不足，则取井水。盖清晨井中
天一之气，浮结于面，以磁器轻取之，殊有补阴之功也。如治火热阳证，则取雪水。能大退
热也。如治伤寒阴证奔豚等疾，则取甘澜水。盖盛之于缸，扬过千遍，水珠沫液，盈溢于面，
其性柔顺，其味甘温，大能和气也。如治脾胃虚弱、泄泻不食等疾，则取池潦水。盖土池
中停蓄既久，不流不动，殊有土气，能助脾元也。如治阴不升，阳不降，乖隔诸疾，则取阴
阳水，河井各半，阴阳相成，可升可降，而使气平者也。
古人用药如羿之射的，不第谙其理，尤贵择其道地者制之尽善。不然，欲以滥恶之剂，
冀其功验，虽扁鹊再起，其可得乎。
凡药有畏恶相反。所谓畏者，畏其制我，不得自纵。如半夏畏生姜之类是也。所谓恶者，
恶其异我，不得自尽。如生姜恶黄芩之类是也。统而论之，彼所畏者，我必恶之。我所恶
者，彼亦畏我。相畏相恶之中，亦有相成者。在因病制方，轻重多寡之间耳。若所谓相反，
则各怀酷毒，两仇不共，共则必害事也。然有大毒之疾，又须用大毒之药以劫之，如古方感
应丸，用巴豆、牵牛同剂，以为攻坚破积之用。四物汤加人参、五灵脂以治血块。二陈汤加
黎芦、细辛以吐风痰。丹溪治尸瘵莲心散，以甘草、芫花同剂，而谓妙处在此。顾良工用之
何如耳。
用药的原则，贵在明白变化。比如气候有古今的差异，地气有南北的分别，天时寒暑交替，人的禀赋有厚薄不同，患病有新旧之分，年龄有老少之别，生活条件有贵贱之差。用药时，不要追求新奇，不要固执一端，不要轻率妄为，不要急于求成。必须慎重精细、圆融灵活。不妨沉下心来仔细体会，以求万无一失，药物才能发挥功效。从前先贤没有阐明这些道理，后学者因此不加以研究，这误人害世不是太多了吗？
疾病有适宜用补法治疗的，可以用泻法来达到补的效果；疾病有适宜用泻法治疗的，可以用补法来达到泻的效果。疾病适宜用寒凉药的，可以用温热药作为引导；疾病适宜用温热药的，可以用寒凉药作为同类相从的引子。病在上部的治下部，病在下部的治上部。病症相同而用药不同，病症不同而用药相同。其中的义理极其精微，学习者最应该深入探究。
用药的禁忌，在于急于求成。急于求成则寒热温凉、行散补泻难免过度。功效尚未显现，危害已经随之而来。药物使用没有次序，就像军队没有纪律，即使有勇猛的将领，也会因为鲁莽而坏事。又如同整理丝线，缓慢则可以理清头绪，急躁则会使结扣更加坚固。
药物有君臣佐使的配合，味道有轻重浓薄的区别，人们都知道这一点。但在实际用药时，让人把药渣再次煎煮，却不知道经过第一次煎沸后，轻且薄的成分已经无味了，重且浓的成分却没有减少。君臣佐使的适宜配伍，究竟在哪里呢？病情浅表的尚且没有大害，病情深重的千万不要这样做。
凡是制作丸剂，必须每种药材单独研磨成细末，根据粉末的重量进行合算，这样分两才准确。否则，容易磨碎的药材一次就磨完了，难碎的药材反复多次也磨不碎。如此粗疏，难怪没有疗效。
凡是苦味的药，直行而泄降；辛味的药，横行而发散；酸味的药，收敛束聚；咸味的药，软坚散结。唯独甘味这一种，可升可降，可浮可沉，可内可外，有和缓之性，有补有泻。大概是因为土味作甘，土位居中央，能兼摄五行。
凡是生于土中的药，上半部分是根，其气上行，治疗中上焦的疾病时使用。下半部分是梢，其气下行，治疗下焦的疾病时使用。出土的植物，上半部分是苗，气味上升。下半部分是茎干，气味守中下达，都适宜。根据病情斟酌使用，使其不违背阴阳规律。
凡是药性作用于上部的，不厌其烦地少量服用。药性作用于下部的，不厌一次性大量服用。少量服用则滋养上部，大量服用则峻补下部。
凡是病在上部的，先吃饭后服药。病在下部的，先服药后吃饭。病在四肢的，宜空腹并在白天服用；病在骨髓的，宜饱食并在夜间服用。
煎药用水，也各有适宜的选择。比如治疗湿肿浮胀的疾病，想要通利水道，就取长流水。因为水流长远，性质通达，能直引至四肢之间。比如治疗大小便不通，以及足胫以下的风湿，就取急流水。因为水流湍急峻烈，性质速下。比如治疗痰饮郁滞，想要吐发升散，就取逆流水。因为水势逆流，回旋涌动。比如治疗中气不足，就取春雨水，有阳气发生之意。比如治疗下元不足，就取井水。清晨井中天一之气，浮结于水面，用磁器轻轻舀取，大有补阴的功效。比如治疗火热阳证，就取雪水，能大退热邪。比如治疗伤寒阴证、奔豚等疾病，就取甘澜水。盛在缸中，扬过千遍，水珠泡沫盈溢于面，性质柔顺，味甘温，大能和气。比如治疗脾胃虚弱、泄泻不食等疾病，就取池潦水。因为土池中停蓄已久，不流不动，具有浓厚的土气，能助脾元。比如治疗阴不升、阳不降、乖隔诸疾，就取阴阳水，河水和井水各半，阴阳相成，可升可降，从而使气机平和。
古人用药像后羿射箭瞄准靶心一样，不仅精通药理，更重视选择道地药材并炮制尽善。不然，想用劣质药剂，期望获得疗效，即使扁鹊再生，可能吗？
凡是有药物相畏、相恶、相反的关系。所谓“畏”，是畏惧对方制约自己，不能放纵。比如半夏畏生姜之类。所谓“恶”，是厌恶对方的异己性质，不能完全发挥药效。比如生姜恶黄芩之类。统而言之，对方所畏惧的，我方一定厌恶它；我方所厌恶的，对方也畏惧我。在相畏相恶之中，也有相互成就的情况。关键在于因病制方，掌握轻重多寡的分寸罢了。至于所谓的“相反”，则是各自怀有酷毒，如两仇敌不能共存，共存则必然害事。然而有大毒的疾病，又必须用大毒的药物来攻劫，比如古方感应丸，用巴豆、牵牛同用，用来攻坚破积。四物汤加人参、五灵脂来治疗血块。二陈汤加藜芦、细辛来吐风痰。丹溪治疗尸瘵的莲心散，用甘草、芫花同用，认为妙处就在这里。只是看良医如何使用罢了。
The principle of medication lies in understanding change. For instance, climate differs between ancient and modern times; geographical qi varies between north and south; heavenly timing alternates between cold and heat; innate constitution varies in thickness or thinness; disease acquisition differs in newness or chronicity; lifespan varies by age (young vs. old); living conditions vary by status (noble vs. humble). When administering medicine, do not seek novelty, do not adhere rigidly to one method, do not act rashly, and do not rush. One must be cautious, precise, flexible, and adaptable. It is advisable to ponder deeply to ensure absolute safety, so that the medicinal efficacy may be achieved. In the past, sages did not elucidate these principles, leading later scholars to neglect them; thus, their error in misleading the world has been significant.
Diseases may require tonification, yet can be treated via purgation methods to achieve tonification. Diseases may require purgation, yet can be treated via tonification methods to achieve purgation. Diseases suitable for cold agents may use hot agents as guiding troops. Diseases suitable for hot agents may use cold agents as similar-guiding引 (guides). When the disease is in the upper part, treat the lower part; when the disease is in the lower part, treat the upper part. The same disease may require different medicines; different diseases may require the same medicine. The meaning here is extremely subtle, and students should investigate it deeply.
The taboo in medication lies in impatience. Impatience leads to excessive application of cold, heat, warm, cool, moving, dispersing, tonifying, and purging methods. Before efficacy is achieved, harm follows immediately. Medicines without order are like an army without discipline; even with brave generals, recklessness will lead to failure. It is like sorting silk threads: slowly, one can clear the ends; hastily, the knots become tighter.
Medicines have Jun-Chen-Zuo-Shi (Sovereign-Minister-Assistant-Courier) roles; flavors have weight, lightness, density, and thinness—everyone knows this. However, when administering medicine, re-decocting the dregs is common, ignoring that after initial boiling, the light and thin components have lost their flavor, while heavy and dense ones remain unchanged. Where, then, is the appropriate balance of Jun-Chen-Zuo-Shi? For superficial diseases, this causes little harm; for deep diseases, one must absolutely avoid this.
When preparing pills, each herb must be ground into fine powder separately, and weights combined accurately. Otherwise, easily crushed herbs are fully processed at once, while hard-to-crush herbs remain incomplete despite repeated grinding. Such negligence explains why there is no efficacy.
Bitter herbs move straight down and drain; pungent herbs move horizontally and disperse; sour herbs bind and astringe; salty herbs stop and soften hardness. Only the sweet flavor can ascend or descend, float or sink, enter inside or reach outside, harmonize or moderate, tonify or purge. This is because Earth's flavor is sweet, Earth resides in the center, and can encompass the Five Elements.
Herbs growing in soil: the upper half above ground is the root, its qi moves upward, used for diseases in the Middle and Upper Jiao. The lower half below ground is the tip/shoot, its qi moves downward, used for diseases in the Lower Jiao. Emergent plants: the upper half is the sprout, its aroma and flavor rise. The lower half is the stem/body, its aroma and flavor stay central and descend downward, both suitable. Adjust according to the disease to ensure harmony with Yin and Yang.
For medicines acting on the upper body, frequent small doses are preferred. For medicines acting on the lower body, single large doses are preferred. Small doses nourish the upper body; large doses strongly tonify the lower body.
For diseases in the upper body, eat first, then take medicine. For diseases in the lower body, take medicine first, then eat. For diseases in the four limbs, take on an empty stomach during the day. For diseases in the bone marrow, take after a full meal at night.
Water for decoction also has specific suitability. To treat damp swelling and floating distension, aiming to promote urination, use long-flowing water. Its nature is penetrating due to its long source, directing flow to the four limbs. To treat obstruction of urine and stool, or wind-damp below the knees, use rapid-flowing water. Its turbulent nature acts quickly downwards. To treat phlegm-fluid retention aiming for vomiting and dispersion, use reverse-flowing water. Its nature flows backward and surges. To treat deficiency of Middle Qi, use spring rainwater, possessing the intent of Yang energy generation. To treat deficiency of the Lower Yuan, use well water. The Tian-Yi Qi in well water floats on the surface in the early morning; scooping lightly with a porcelain vessel greatly benefits Yin tonification. To treat Fire-Heat Yang syndromes, use snow water, which greatly reduces heat. To treat Cold Damage Yin syndromes or Ben-Tun (running piglet), use Gan-Lan Water (sweet ripple water). Stirred vigorously over a thousand times in a vat until foam covers the surface, its nature is soft and smooth, flavor sweet and warm, greatly harmonizing Qi. To treat Spleen-Stomach weakness, diarrhea, and anorexia, use pond rainwater. Stagnant in earthen ponds for long periods, unmoving, it possesses strong Earth Qi, assisting Spleen Yuan. To treat disorders where Yin does not ascend and Yang does not descend, causing separation, use Yin-Yang Water (half river water, half well water). Yin and Yang complement each other, capable of ascending and descending, thereby calming Qi.
Ancients used medicine like Yi shooting at a target: not only mastering theory, but especially selecting authentic regional materials and processing them perfectly. Otherwise, expecting efficacy from inferior agents, even if Bian Que were reborn, would it be possible?
Herbs have relationships of Fear (Wei), Dislike (E), and Opposition (Xiang Fan). "Fear" means fearing control by another, preventing free action, e.g., Pinellia Rhizome (Banxia) fears Ginger (Shengjiang). "Dislike" means disliking heterogeneity, preventing full efficacy, e.g., Ginger (Shengjiang) dislikes Scutellaria Root (Huangqin). Generally, what one fears, the other dislikes; what one dislikes, the other fears. Within mutual fear and dislike, there are also mutual achievements. It depends on formulating prescriptions based on disease and balancing weight and quantity. Regarding "Opposition," each harbors harsh toxicity; like two enemies, they cannot coexist, and coexistence causes harm. However, for severe toxic diseases, highly toxic drugs must be used to seize control, e.g., the ancient formula Gan-Ying Wan uses Croton Seed (Badou) and Morning Glory Seed (Qianniu) together to break accumulations. Si-Wu Tang added with Ginseng (Renshen) and Trogopterus Dung (Wulingzhi) treats blood masses. Er-Chen Tang added with Veratrum Rhizome (Lilu) and Asarum Root (Xixin) induces vomiting of wind-phlegm. Danxi’s Lotus Heart Powder for Corpse Consumption (Shizhai) uses Licorice Root (Gancao) and Genkwa Flower (Yuanhua) together, claiming the miracle lies here. It ultimately depends on how skilled physicians apply them.