What in the plant known as monkshood, devil's helmet or blue rocket is sometimes used medicinally?

Question

First used in western medicine in the 17th century what was the drug developed from the bark of the cinchona tree used to treat?

Answer 1

Aconitine
The plant, and its alkaloid aconitine, is also highly poisonous. The difference between its being medically helpful and being deadly is very small.