Pharmacognosy is the study of biologically active compounds found in plants and animals, many of which have been used for thousands of years in traditional medicine and now form the basis for modern pharmaceuticals. Approximately 25% of medications originate from natural sources. Secondary metabolites produced by organisms often serve defensive functions but can have pharmacological effects in humans when isolated and dosed properly, such as hypericin from St. John's Wort for mood enhancement, echinacoside from Echinacea for immunity, and digoxin from foxglove to regulate heart rhythm. While isolated compounds allow for mass production and standardization, crude plant and animal materials may provide synergistic effects from multiple constituents.