Mefloquine insusceptibility of malaria in Thailand not promoted by nonregulated drug-use.

To determine whether the nonregulated use of antimalaria drugs on the Thai-Cambodian border promoted the recent loss of mefloquine sensitivity by Plasmodium falciparum parasites, we identified the drugs consumed there by gem miners. Of those drug concoctions that were self-administered, five contained chloroquine, one contained primaquine and bactrim and two merely contained analgesics. Local nonregulated healers prescribed orally administered concoctions containing chloroquine, quinine, tetracycline or primaquine for patients suffering mild illness. They administered parenteral chloroquine for severely ill patients and quinine when malaria was life-threatening. Certain of these healers prescribed fansimef, but only infrequently and after 1990. Because gem miners appear not to have had informed access to mefloquine before 1990, we conclude that the loss of antimalaria efficacy of this drug on the Thai-Cambodian border during the late 1980s was independent of its nonregulated use.