Article snapshot taken from Wikipedia with creative commons attribution-sharealike license.
Give it a read and then ask your questions in the chat.
We can research this topic together.
Cymarin (or cymarine) is a cardiac glycoside. Apocynum cannabinum used as a source of fiber by Native Americans and Apocynum venetum (Chinese: 羅布麻) used as a herbal tea in China both contain cymarin. Cymarin is a cardiogenic toxin that causes cardiac arrhythmia in humans. Cymarin is used in modern medicine in formulated pharmaceutical preparations used to treat a variety of tumors.
References
Edible and Medicinal plants of the West, Gregory L. Tilford, ISBN 0-87842-359-1