This formula is basically Yan Hu Suo with one added herb, Bai Zhi. The addition gives it a little extra “kick” to move it upwards and and outwards and into the smaller collaterals. For that reason it is often used for headaches and upper spine.
Yan Hu Suo (Corydalis yanhusuo) is an herb that is effective for pain. Apparently scientists at UC Irvine have found that the roots of Yan Hu Suo (which has been used for thousands of years for pain in China) contains a compound called dehydrocorybulbine (DHCB). Who knew!? They call this type of research Neuropharmacology. This research was done on rodents so animals were harmed, so be forewarned.
Bai Zhi (Radix Angelicae Dahuricae)