Is断阳草 (duàn yáng cǎo) the same as coriander?
Generally speaking, Duan Yang Cao (断阳草) is not cilantro (coriander), and there is no medical terminology in which cilantro is referred to as Duan Yang Cao. Detailed analysis is as follows:

Duan Yang Cao is the dried root or the xylem of the root of Tripterygium wilfordii, a plant from the Celastraceae family. It is characterized as cold in nature with a bitter and pungent taste and is highly toxic. It is primarily used for eliminating wind-dampness, promoting blood circulation to unblock meridians, reducing swelling, and relieving pain. However, it must be used with extreme caution due to its toxicity, which may seriously affect health. On the other hand, cilantro is an annual herb belonging to the Apiaceae (Umbelliferae) family and the Coriandrum genus. It is warm and pungent in nature and mainly affects the lung and stomach meridians. It has functions including relieving exterior syndromes, promoting eruption of rashes, strengthening the spleen and improving digestion, promoting diuresis and bowel movements, refreshing the mind, and alleviating pain. These two are definitely not the same thing.
To ensure safe and effective usage, it is recommended to use or discontinue Duan Yang Cao under the guidance of a qualified traditional Chinese medicine practitioner. Additionally, dietary regulation should be emphasized during treatment, excessive fatigue should be avoided, and good lifestyle habits should be maintained to promote recovery.