Wonder Drugs Waiting in the Weeds?

John Roach
for National Geographic News
May 3, 2005
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."


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.

But Stepp argues that the backyards of indigenous peoples are more likely sources. He said that if a pharmaceutical researcher gave indigenous villagers a list of a hundred illnesses and asked them to find plant remedies for each, "they'd spend most of the time right down the trail."

The weeds and other plants that grow near indigenous villages are the most well known and widely used, Stepp said. "Are there likely [medicinal] plants deep in the jungle? Yeah, there's a good chance of it. But it's also likely that no one else [including indigenous peoples] knows about it," he said.

The properties that make a weed a weed, and thus able to thrive in disturbed habitats, are what make the plants potentially beneficial.

Weeds are full of bioactive compounds. These are chemicals that weeds have evolved over thousands of years to compete against other plants and to prevent predators, such as insects, from munching their leaves.

These compounds "are toxic to predators. But [when] used in the right dose, they have therapeutic benefits for humans," Stepp said.

Coley, the University of Utah biologist, said competition between plants and insects have also forced slow-growing, nonweedy tropical plants to evolve chemical defenses.

Study Implications

Meanwhile Stepp said his 2004 study results suggest that, in addition to exploring the rain forest for medicinal plants, it makes sense to look at the 8,000 or so weeds that grow closer to home.

The anthropologist doesn't think drug companies share that view. "The difference between wanting to find new medicines and wanting to find new medicines to make a profit is a very wide gulf," he said.

Stepp said that to profit from a plant-derived drug, pharmaceutical companies need to patent new compounds. But the chemistry of most weeds is already described and in the public domain, he noted.

Regardless of whether new drugs are developed from weeds, Stepp said weed research could lead governments to respect and promote traditional medicinal practices, especially in parts of the world with limited access to modern health care facilities.

"In Western, industrialized countries, healing is very much a specific act. You are taught to not self-medicate. If you're sick, you go to a doctor, and they'll tell you what's wrong," he said. "That's not true for the vast majority of the world. They do treat themselves."

Stepp and his colleagues published their study in the June 2004 issue of the Journal of Ethno-Pharmacology.

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