The crude drug Aconite Root (bushi in Japanese) contains toxic compounds called bushi diester alkaloids(BDA), and the raw material with high BDA content has been considered suitable for processing into prepared bushi. Moreover, processing methods and an upper BDA content limit are prescribed in the 16th edition of Japanese Pharmacopoeia. In this study, we closely examined descriptions of high quality bushi in ancient herbal literature, and elucidated the relationship between bushi forms and BDA contents. The results showed that large-sized bushi with enlarged rootlets called “horns” (tsuno in Japanese) were considered higher quality, and the horns and the root tops (hozo in Japanese) were cut off when used as bushi. Meanwhile, chemical studies have shown that larger bushi contains less BDA, and the root tops and the horns contain more BDA than the root body. These results suggest that selecting larger roots and cutting off the more potent parts were processes for reducing BDA. Therefore we conclude that consistently less toxic bushi was considered a higher quality product in ancient times.