Scientists have finally pinned down the molecular mechanism by which castor oil works, according to a report in the Proceedings of the National Academy of Sciences (DOI: 10.1073/pnas.1201627109). Despite centuries of use to relieve constipation and induce labor in pregnant women, castor oil has remained a mystery. Researchers knew that the active compound is a hydroxylated C18 fatty acid, known as ricinoleic acid, that is released from castor oil triglycerides by intestinal lipases. But what happens to the ricinoleic acid after has been unknown. Now, a German research group led by Stefan Offermanns, director of pharmacology at the Max Planck Institute for Heart & Lung Research, has found that ricinoleic acid exerts castor oil’s physiological effects by interacting with a receptor in intestinal and uterine smooth-muscle cells. The receptor, EP3, is one of four for prostaglandin E2. This prostaglandin is known to soften the cervix and cause uterine contraction and is marketed as a drug to induce labor. The specificity of ricinoleic acid activity is remarkable, the authors say, because castor oil has generally been assumed to work through nonspecific mechanisms.