Artemisinin drugs are a class of anti-malarial drug derived from a Chinese medicinal herb. Although also sold commercially in combination with other anti-malarials, about 60% of sales are "stand-alone" artemisinin drugs (monotherapy), particularly in Africa. This drug acts by decreasing the number of Plasmodium parasites in the blood by up to a factor of 100 million, and can even cure the commonly-fatal cerebral malaria.

To completely kill the parasite, however, artemisinin compounds must be taken alone for a prolonged period, or for three days in combination with another antimalarial. The instructions that come with artemisinin-only pill packs often say the drugs are best taken with other antimalarials, but they are regularly sold alone. Frequently, only one or two days' worth are sold, enough to make many patients feel better but sometimes too little to achieve a cure.

Although no truly artemisinin-resistant strains have been found in nature, strains with decreased sensitivity have been identified in the lab. To prevent the possible emergence of resistant strains of Plasmodium (particularly, P. falciparum), WHO called on 17 pharmaceutical companies recently to stop selling the drug as a stand-alone therapy against malaria.

"WHO is taking this preemptive action in order to protect this medicine from becoming ineffective," the head of the organization's malaria department, Arata Kochi, announced at a news conference in Washington. If millions of cases of malaria continue to be treated with artemisinin alone, "the potential risk is tremendous," he said.

Read more from the Washington Post.

For supplemental reading: A dissertation, Studies on molecular mechanism of action and synthesis of new derivatives of the antimalarial drug artemisinin (published March 2005), is available for download from Amazon.