Cold bitter da huang belongs to the west, autumn, and dryness.
It strongly cools and descends the yang qi in the stomach domain. Which is why it is used in the Cheng qi family of formulas.
It is used when internal yang ming heat is so great that the normal cooling, drying, and descending function fails.
But there is another use of da huang that is not so well known. And that is in clearing evil qi from the channels.
By descending, cooling, and drying the stomach domain the channels are affected. Which is why da huang can cause problems if used improperly.
Plain Questions Chapter 5
The six channels are streams, the intestines and the stomach are the sea. The nine orifices are where the qi flows like water.
Zhang Jiebing suggests that the last sentence should be; The nine orifices are where water and qi flow.
All rivers flow into the seas.
By descending the yang ming the sea is drained and the channels which are rivers follow.
The most representative formula of this action is tao he cheng qi tang.
Tao he cheng qi tang
tao ren 9 da huang 12 gui zhi 6 mang xiao 6 zhi gan cao 6
When a tai yang disease is unresolved and heat binds in the bladder, the person is as if manic, and spontaneous blood descent will bring recovery. If the exterior has not been resolved, one should not yet attack, but should first resolve the exterior. When the exterior has been resolved and there is only tense bound lesser abdomen, one can attack and therefore tao he cheng qi tang is appropriate.
It treats blood build up in the lower abdomen which is caused by heat. There will be acute lower abdominal pain, incontinence of urine, night fevers, mental vexation, and thirst. The heat can arise from an external tai yang pattern that progresses to the interior and turns into heat. The heat can be in the lower abdomen, body, or bladder.
The internal heat build up in the lower abdomen is cleared by moving the blood with tao ren and gui zhi, and by draining the seas with da huang and mang xiao.
This is also being done in Feng yin tang.
Feng yin tang
da huang 9 gan jiang 9 long gu 9 gui zhi 6 zhi gan cao 3 mu li 3 han shui shi 12 hua shi 12 chi shi zhi 12 bai shi zhi 12 zi shi ying 12 shi gao 12
Chapter 5 Feng yin tang: for heat paralysis and epilepsy.
Feng yin tang eliminates heat that causes paralysis and seizures. It treats seizures due to major wind in adults, and in children treats convulsions or seizures due to fright, occurring multiple times a day, and medicine has not been able to treat, for it is a heat eliminating formula.
It clears excess heat and internal wind in acute patterns.
Da huang belongs to the west, autumn, and dryness. It also drains the seas to drain the rivers.