The US Centers for Disease Control and Prevention (CDC) and most cli nicians state that chloroquine, not hydroxychloroquine, generally is preferred for the prevention and treatment of malaria caused by Plasmodium susceptible to 4-aminoquinoline derivatives. (See Uses: Malaria, in Chloroquine 8:30.08.) However, hydroxychloroquine is as effective and may be better tolerated and may be used when chloroquine is unavailable