Searching through scientific journals, anthropologist John Richard Stepp, from the University of Florida, found that although only about 3 per cent of the world's quarter-million plant species are weeds, they make up more than a third - 36 - of the 101 plant species used in pharmaceuticals.
"If I had one place to go to find medicinal plants, it wouldn't be the forest. There are probably hundreds of weeds growing right outside people's doors they could use," he said.
Perhaps the world's best-known medicinal weed is the poppy, from which morphine is derived, Stepp said. Scopolamine, an important drug for treating motion sickness, also is weed-based, as are the cancer medicines vinblastine, for Hodgkin's disease, and vincristin, for childhood leukemia, he said.
"With all the emphasis on the tropical rain forests, an entire area is being missed in natural products research," said Stepp, whose results appear in this month's issue of the Journal of Ethno-Pharmacology (Vol 92, issues 2-3, pp 163-166).
While Stepp studied plant-derived pharmaceuticals, based on single compounds extracted from a plant, rather than herbal medicines like ginkgo biloba and garlic, his research idea came from observing residents in the southwestern Mexican state of Chiapas.
For the vast majority of the 800,000 Mayans living in this area, medicinal plants are the primary source of health care, Stepp said. "What people don't realize is that two-thirds of the world's population relies on plants in this way," he noted.
In a further experiment comparing plants picked from fields in Georgia, US with those gathered from a forest, Stepp and an undergraduate team found that as many as 50 plants from the fields had been used medicinally by Native Americans compared with only 12 of those they gathered from the forest.
"The realisation that medicinal plants are readily available in a living pharmacy right outside the door and along the sides of trails rather than deep in the forest could lead governments to encourage and promote traditional medicinal practices," Stepp said. "They are readily available, cost nothing to gather and are often more effective."