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(Attached: Bitter Taste in Mouth, Sweet Taste in Mouth)
Thirst is a critical key point in Shanghan (Cold Damage diseases) and must be carefully observed. When wind-cold pathogens are at the exterior, affecting the Taiyang channel, thirst is typically not mentioned. Once the pathogen enters the Yangming stage, the patient ceases to fear cold and instead fears heat, gradually experiencing thirst in the mouth. In cases where thirst appears immediately at the onset of a Taiyang disease, it indicates warm-heat pathogens emerging from the interior to the exterior; although exterior symptoms are present, the pathogen is not truly at the exterior. Thus, Zhang Zhongjing stated: "Taiyang disease with fever and thirst, without aversion to cold, is classified as Warm Disease." Additionally, patients suffering from Heatstroke (Zhong Shu) exhibit sweating, aversion to cold, body heat, and thirst; this condition is unsuitable for diaphoresis (sweating therapy) and requires treatment different from wind-cold. Regarding Damp-Warmth at its onset, when dampness has not yet transformed into heat, the mouth may feel dry but the patient cannot drink; only after dampness transforms into heat does severe thirst with an urge to drink large amounts occur. Therefore, by observing whether there is thirst and its severity, one can distinguish the location of the pathogen (exterior vs. interior) and the nature of the middle jiao (cold vs. heat), thereby assessing the intensity of the heat. This differentiation should be the primary focus during clinical practice.

For all Yang-heat syndromes, once thirst appears, the primary treatment should be to drain heat. In Yin syndromes (San Yin diseases), if the condition arises from intrinsic deficiency-cold of the organ itself, thirst naturally does not occur. However, when a Yin syndrome transforms into a Yang syndrome, fever accompanied by thirst may appear. In such cases, one should harmonize the fluids rather than directly suppress the heat. Since Yin excess and Yang deficiency are present, thirst is actually a sign of Yang recovery; draining heat would further deplete Yang. If dryness and thirst intensify due to the depletion of Yin fluids, while draining heat, one must especially assist with nourishing Yin and generating fluids. This principle applies to those with severe thirst and an urge to drink. Conversely, if the mouth feels dry but the patient refuses to drink or avoids water, this indicates a pattern of True Cold and False Heat. The pulse may be deep and fine or empty and large. One should use warming agents to guide Yang back to its root, revealing the true state of deficiency-cold. Mistakenly using cold-cooling herbs would lead to rapid disaster.

Thirst is commonly seen after Huoluan (Vomiting and Diarrhea). The Classics state: "Huoluan with headache, fever, and body pain; if heat predominates and the patient desires water, Wuling San (Five-Ingredient Poria Pill) is indicated. If cold predominates and the patient does not desire water, Lizhong Wan (Regulate the Middle Pill) is indicated." This is because vomiting and purging damage the body fluids, and as Yin transforms to Yang, regulating the Spleen is appropriate to allow the Spleen Qi to distribute essence, resolving both exterior and interior aspects, circulating fluids, and stopping thirst. One must not blindly administer cooling herbs upon seeing thirst, as this may worsen the condition. Furthermore, Wuling San raises Spleen fluids and unblocks the water passages. If Taiyang disease remains unresolved and the pathogen invades the Bladder Fu organ, presenting with both exterior and interior symptoms, thirst, desire to drink, and vomiting after drinking, this formula is also indicated. It should be taken with rice water, one cun-bi dose, followed by warm water to induce sweating for cure. If there is no Taiyang exterior syndrome, but only floating pulse, fever, thirst, and difficult urination, the warming transformation of Wuling San is inappropriate; instead, Zhuling Tang (Polyporus Decoction) should be used to clear heat and regulate fluids. Such cases differ from treating Yang-heat thirst with sweet-cold herbs.

Generally, thirst belongs to interior syndromes. If heat is in the channels but not in the interior, the mouth may feel dry, but the patient only wishes to rinse their mouth with water without swallowing. This is a precursor to epistaxis (nosebleed) or maculopapular rashes. One should not hastily use cold-cooling herbs that might obstruct the pathogen; instead, analyze the cause and apply exterior-resolving methods. For various thirst conditions where the patient desires water, provide small amounts frequently until Stomach Qi harmonizes and the condition resolves. Excessive water intake may lead to complications such as asthma, hiccups, palpitations, and fullness; caution is essential.

Bitter taste in the mouth occurs when heat pathogens enter the Shaoyang channel; thus, Zhang Zhongjing lists bitter taste and dry throat as the guiding symptoms of Shaoyang disease. Sweet taste in the mouth is termed "Pi Dan" (Spleen Heat/Dampness) in the Inner Canon, caused by damp-heat stagnation and steaming. It is appropriate to clear stale qi from the chest. Although neither condition involves thirst, both belong to heat syndromes, and acrid-warm herbs are contraindicated.