Castor is a plant that produces seeds (beans). Castor oil is produced by pressing ripe seeds that have had their outer covering (hull) removed. The hull contains a deadly poison called ricin. Castor oil has been used as medicine for centuries.
Castor seeds without the hull are used for birth control, constipation, leprosy, and syphilis.
Castor oil is used as a laxative for constipation, to start labor in pregnancy, and to start the flow of breast milk.