Many drugs were first discovered in plants or microorganisms. These are alternative sources, not consecutive stages. Scientists may extract useful chemicals from plants, such as digitalis from foxgloves or a chemical from willow bark that led to aspirin. Alternatively, they may discover useful chemicals made by microorganisms, such as penicillin made by Penicillium mould.

A possible development pathway is: plant source -> chemical extraction, or microorganism source -> discovery of a useful chemical. A promising chemical is then tested in the laboratory and on animals before clinical trials. In clinical trials, volunteers or patients are randomly allocated to receive the real drug or a control treatment. Treatments are compared while double blinding is maintained during treatment and outcome recording, so neither the participants nor the researchers recording the outcomes know who received which treatment. Different doses may be tested to find the optimum dose before the drug is approved for use.