Plants have long been a source of pharmaceutically useful compounds and many medicines trace their discovery to the study of plant natural products. Today more than 130 products include plant derived compounds as active ingredients. However, only a small percentage of plant species have been assayed against a small number of therapeutic targets to discover the known plant-derived drugs. A series of calculations will be reviewed to elucidate the known number of plant species, estimates of the percentage that have been evaluated, and possible estimates of the numbers of pharmaceutically useful compounds that remain to be discovered. There are certainly more than 300,000 species of vascular plants. It is less obvious what percentage have been evaluated, as most that have been studied have been assayed against only limited numbers of therapeutic targets. It is clear from these calculations that only a small percentage of plant-derived drugs have been discovered and that the number of potential new discoveries is vast. The constraints on future discovery will also be reviewed to better understand the obstacles that exist for the discovery of new plant-derived drugs.
