It's often said that plants hidden in the tangle of the Amazonian rain forest may harbor an undiscovered cancer cure. John Richard Stepp thinks the same can be said for the world's weeds.

Stepp is an enthobiologist, a scientist who blends anthropology and biology to study plant use by different cultures.

In a study published last year, Stepp, a professor at the University of Florida in Gainesville, looked at the ingredients of common drugs sold at local pharmacies. Of the 101 primarily plant-based health remedies he examined, Stepp estimated that the main ingredient in about a third of the drugs was a weed.

"Those are fairly conservative numbers too," he added. "It is actually more than a third. We cut quite a few off [the list], because they weren't quite weedy enough, in my view."

To be "weedy enough" to meet Stepp's criteria, plants had to be fast-growing and soft-tissued—unlike trees, for example, which are slow-growing and have woody tissues. Stepp also limited his definition of weeds to plants that thrive in disturbed areas, like farms or clearings, without being deliberately planted.

Examples of well-known weed-derived drugs include the painkillers morphine and codeine, which are made from a poppy (Papaver somniferum). Another remedy is the motion sickness drug scopolamine, which is made from devil's trumpet (Datura metel).

Stepp said his tally of common weed-based drugs was ten times higher than expected. There are about 8,000 weeds known to science. That figure comprises only about 3 percent of the world's 250,000 described plants.

Phyllis Coley is a biologist at the University of Utah in Salt Lake City and the Smithsonian Tropical Research Institute in Panama. She conducts drug-discovery research in the Panamanian tropical rain forests. She said Stepp's finding is "provocative."

Coley noted that weedy plants have long been a source of traditional medicines. A key question, she said, is, Why?

"Perhaps this is because [weeds] were shown to work better," she said. "But I would also suggest that weedy plants would be much more readily available, as they grow in association with human disturbance and [are] therefore more likely to be experimented with [and] developed into medicines."

"Bio-prospecting"

In recent years conservationists and pharmaceutical companies have championed indigenous peoples as sources of medicinal plant knowledge. A popular notion is that indigenous peoples can lead researchers to new drugs that are, for example, hidden deep in tropical rain forests.

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SOURCES AND RELATED WEB SITES Pulse of the Planet University of Florida: John Richard Stepp University of Utah: Coley/Kursar Lab