Patients feel better after just a day or two of the drug, and either seek out sellers who will dispense only a few pills or simply stop taking it, setting themselves up for a harder-to-treat relapse.

The fear is that improper use of artemisinin will spur the malaria parasite to evolve to resist treatment with the compound, something that has happened to all previous malaria therapies.

"If we lose artemisinin, we will no longer have an effective cure for malaria," said Dr. Arata Kochi, WHO's malaria chief. "It will take at least 10 years before another effective drug will become available."

One of the world's biggest drug companies, France's Sanofi-Aventis, immediately signaled that would it comply with WHO's unusual public complaint.

Trying to Shame Drug Makers

Malaria sickens up to half a billion people a year, and kills more than 1 million, many of them young children.

Forms of artemisinin, developed in China from the wormwood plant, have become the leading treatment -- but only when used together with older medications. This artemisinin combination therapy, known as ACT, is 95 percent effective in curing malaria.

Taking artemisinin drugs alone, or so-called monotherapy, will make resistance appear more quickly, leading many groups that fund malaria treatments in developing countries to now pay only for ACTs.

The problem: In many of those countries, patients can buy artemisinin monotherapy from private doctors or, worse, in village marketplaces -- and it often comes with manufacturer-written instructions to take for only five days, WHO found. Resistance aside, five-day treatment with artemisinin alone isn't enough to cure anybody.

More frightening is that patients feel better after just a day or two of the drug, and either seek out sellers who will dispense only a few pills or simply stop taking it, setting themselves up for a harder-to-treat relapse.

The WHO can't stop these private practices. But Thursday, frustrated that companies had ignored more diplomatic requests to halt monotherapy sales, the UN agency took the unprecedented step of essentially trying to shame drug makers into drying up the monotherapy supply.

Halt Monotherapy Marketing

WHO said if 18 manufacturers hadn't halted monotherapy marketing within three months, it would consider stronger options such as asking governments to curb manufacture and shipment of the products. The agency praised Sudan for already banning importation of artemisinin monotherapy.

Most of the companies are fairly small manufacturers in China, India and Vietnam. But also on the list is drug giant Sanofi-Aventis, which said that it already had begun phasing out monotherapy sales but now would speed that process.

"Scientifically, it's a good decision because the risk of resistance is obvious," said Sanofi's Dr. Robert Sebbag, noting that Sanofi also manufactures ACT, making the change easy.

Using the safer and more effective combination therapy will add only about 15 cents to the cost of treatment, said WHO's Dr. Andrea Bosman. If bought through public programs run by WHO or other groups, full ACT treatment can cost between $1.30 and $1.55, compared with $1.15 to $1.40 for monotherapy, he said.

Even if companies do quit specifically marketing monotherapy, ACTs currently come in blister packs that package artemisinin and older medicine together. Patients or drug sellers simply could cut out just the ones they want, said malaria specialist Nick White of Thailand's Mahidol University.

"This is definitely an interim solution," White said of WHO's call. He noted that later this year, simpler ACT formulations that combine artemisinin and older medicines into the same pill should begin sales.

Some are predicted to sell at half the price of today's ACT versions.