A new drug may be "discovered" by numerous methods, as listed below.
Testing traditional or folk medicine. The root of the plant Podophyllum peltatum was traditionally used by North American aboriginal people to treat warts. The active chemical, called podophyllotoxin, was isolated in 1940, and was found to have anticancer activity. Etoposide, a semisynthetic chemical made from podophyllotoxin, is used in the treatment of some cancers.
Accident or serendipity. The anticancer activity of the chemicals found in the rosy periwinkle (Catharanthus roseus) was discovered accidentally. Native people in Africa, Australia, India, and South Africa traditionally used the rosy periwinkle to treat diabetes. Two scientists tested the extract to find out whether it was useful in lowering blood sugar, and found that it was not. Something in the extract was found to reduce the number of white blood cells in test animals, however, and two chemicals, called vincristine and vinblastine, were isolated and are still used as anticancer agents in the treatment of leukemia today.
Random sampling of chemicals. After the discovery of penicillin by Dr. Alexander Fleming in 1928, the screening of chemicals found in nature became an important way to find new drugs.
Rational drug design. Cimetidine is a chemical that is used to treat stomach ulcers. It is not known how ulcers start, but it is known that the amount of acid in the stomach makes the ulcer worse. To design cimetidine, chemists first had to understand how the stomach produces acid. They then had to design a chemical that blocks the acid production so that the stomach has a chance to heal. Cimetidine was the result of rational drug design.