Although we also use medicines derived from fungi, lichens, and actinobacteria, seed plants are the source of many of our medications. A few examples of medicines derived from plants are shown in Table 28.1. Even in synthetic pharmaceuticals, the chemical structures of the active ingredients are often based on the biochemistry of substances isolated from plants.

How are plant-based medicines discovered? Many were discovered over the millennia by people who lived alongside useful plants and discovered the properties of the plants through trial and error. Some plant-based medicines have come into widespread use through the work of ethnobotanists, who study how people use and view plants in their local environments. This work proceeds all over the globe today. An older example of this approach is the discovery of quinine as a treatment for malaria. In the sixteenth century, Spanish priests in Peru became aware that the native population used the bark of local Cinchona trees to treat fevers. The priests successfully used the bark to treat malaria. Word of the medicine spread to Europe, where it was put into use as early as 1631. The active ingredient of Cinchona bark—quinine—was identified in 1820, and quinine remained the standard malarial remedy well into the twentieth century.

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Since the mid-1900s, many plant-based medicines have been found by systematic testing of plants from all over the world. One example of a medicine discovered in this way is taxol, an important anticancer drug. Among the myriad plant samples that had been tested by 1962, extracts of the bark of the Pacific yew (Taxus brevifolia) showed anti-tumor activity in tests against rodent tumors. The active ingredient, taxol, was isolated in 1971 and tested against human cancers in 1977. After another 16 years, the U.S. Food and Drug Administration approved it for human use, and taxol is now widely used in treating breast and ovarian cancers as well as several other types of cancers.