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The 《金匮要略》has a chapter on 痰饮 where this prescription appears twice.

1. To treat 痰饮：

“先渴后呕，为水停心下，此属呕家，小半夏茯苓汤主之。”

The explanation: Normally, you first vomit, then feel thirsty due to the lost of fluids. When you drink and then vomit, it’s because you fluids are not being transported around as they should be. Instead, they get stuck below the heart (where the stomach is), the result of which is that anything else (water for example) which goes in causes vomiting.

We understand that 小半夏汤 is used to treat the vomiting. Why add poria (茯苓) here? Poria is to treat the 痰饮, to get rid of stagnating fluids ‘below the heart.’

2. To treat 支饮：

“卒呕吐，心下痞，膈间有水，眩悸者，小半夏加茯苓汤主之。”

The explanation: Here, he vomits suddenly. And also has a bloated feeling around the stomach area with watery feeling through the hypochondriac area. Dizziness and palpitations accompany. Through reasoning, one can imagine how this case of vomiting is much stronger than the situation mentioned above, so severe that he gets dizzy and palpitations, probably his cardiovascular system adjusting to the sudden lack of fluids.

We understand that 小半夏汤 is used to treat the vomiting. But why add poria (茯苓) here? Poria is to get rid of the excess fluids, which here are considered 支饮.

A case of heat stroke:

“In a case study from 《谢映庐医案·第一卷》published by the 上海科学技术出版社 in 1962, we have a patient who drinks too much water on a dry and hot summer day. His chest begins to feel pain, he perspires exaggeratedly, and vomits to no end. When the doctor gets to ask him questions, he reveals that he is not thirsty, his pulse is not dry, and so gives him warm herbs for the stomach. The pain the chest begins to abate, but the vomiting continues, and so does the perspiration and accompanying dizziness.

Astragalus (黄芪) is added to the original formula, and still no results area reaped upon drinking it - the perspirations continues, and so does the dizziness and palpitations. The doctor who accepts this patient has only one last try, and he uses 小半夏加茯苓汤. Soon after, the perspiration and vomiting stop, and so does the dizziness and palpitations.”

The TCM vantagepoint?

The symptoms seem to tell us that the guy is replete with fluids. His body’s rejection of any oral fluid intake confirms this - “we’ve got enough water in the body, stop pumping more in!” So poria is useful for getting rid of water, while pinella and ginger are good for stemming the vomit reaction.
I don’t buy this totally.

How do we understand this?

I think that the patient got a moderate case of heat stroke, which affects regulation of the following:

1. temperature (this especially)
2. sweat glands
3. blood vessel activity

The excess water being taken in causes blood osmotic pressure to fall, encouraging flow of water into the tissues. Blood pressure falls as the result. So the dizziness and palpitations are not surprising.

Sweat glands go crazy from the heat and seek to get rid of the water. The temperature control center is also kinda messed up, and keeps seeking to take away excess heat. Hence the perspiration and vomiting.

This guy’s body is in alert mode and is doing all it can to reduce heat via perspiration. It is also averse to water intake. Giving warm herbs for the stomach, especially tonics like 黄芪 may not be the answer yet, especially if the herbs are warm and dehydrating.

A better solution would have been to give the guy fluids rich in minerals, hence putting his electrolyte system in order. But if herbs were the only resource available, the formula makes sense.

1. First, inhibit your vomit center by using Pinella (半夏) and ginger (生姜)
2. Poria (茯苓) is likely not used as a diuretic here. It probably provides a gentle way to build up and regulate a demoralized immune-physiological system.