Phytochemical screening refers to the extraction, screening and identification of the medicinally active substances found in plants. Some of the bioactive substances that can be derived from plants are flavonoids, alkaloids, carotenoids, tannin, antioxidants and phenolic compounds. Although the knowledge of how these substances provide medicinal value to humans reflects a relatively recent scientific understanding, the use of plants and plant extracts to heal, relieve pain and promote good health dates back to before the beginnings of medical science.
It is believed that there may be about 4,000 phytochemicals contained in plants that can be used to prevent, minimize or remedy medical conditions such as strokes, cancer or metabolic syndrome. The evidence obtained through current scientific research does not appear to demonstrate that the use of phytochemical supplements supports long-term health as well as consuming the actual fruits, grains and vegetables from which they were taken. The long-term use of phytochemical supplements as a substitute for their natural food sources should only be considered after consulting a doctor, as noted by the American Cancer Society.