This natural extract of the plant Artemisia annua (qing hao, sweet wormwood) is a sesquiterpenelactone peroxide that is structurally distinct from all other known antiparasitic compounds. Extracts of qing hao were recommended for the treatment of fevers in China as early as AD 341; their specific antimalarial activity was defined in 1971. Although qinghaosu has also been shown to be active against the free-living ameba Naegleria fowleria and several trematodes, including Schistosoma japonicum, Schistosoma mansoni, and Clonorchis sinensis, its greatest impact to date has been in the treatment of malaria. Extensive investigations showed it to be schizonticidal for both chloroquine-sensitive and chloroquine-resistant strains of P. falciparum. Several derivatives, among them artemether†and artesunate, are significantly more active than the parent compound. All are concentrated in parasitized erythrocytes where they decompose, releasing free radicals, which are thought to be damaging to parasitic membranes. Artemisinin compounds act more rapidly than other antimalarial agents, stopping parasite development and preventing cytoadherence in falciparum malaria. Although depression of reticulocyte counts has been noted, these agents appear significantly less toxic than quinoline antimalarials. As there is some evidence that they may possess teratogenic properties, they should not be used in pregnancy. Importantly, they may be given orally, rectally (by suppository), and parenterally. Relapses can occur unless they are given for several days or combined with a second agent such as mefloquine or tetracycline.