This formula is a combination of two of the most famous formulas in Chinese herbal medicine. Xiao Chai Hu Tang comes from Zhang Zhong-jing's Shan Han Lun (Treatise on Damage [Due to] Cold). Si Wu Tang comes from the Tai Ping Hui Min He Ji Chu Fang (Tai Ping [Era] Imperial Grace Formulary), a famous formulary published in the Song dynasty. A 10:1 extract.