Strophanthus kombe/hispidus

This tropical liana grows in Africa and was quite familiar to native tribes that had used it widely through the centuries. 

Native tribes obtained a thick liquid from its seeds, containing the poison. They dipped the end of the arrows in it, so they were even able to kill an animal sometimes. 

Europeans got acquainted with the plant after botanist Dr. Kirk brushed his teeth one fateful morning… He was particularly interested in the poisonous weapons of the tribes and collected arrows in a bag, where he kept personal belongings such as his toothbrush for example. The collection grew large enough to be placed elsewhere. That particular morning, he felt a strong heartbeat and concluded it had been because of the arrows. 

In 1885, the English investigator Fraser extracted a glycoside named strophantin, which quickly found application in cardiology, but it has now been replaced by more sophisticated medicines.