Dictionary.net

A biguanide compound which has little antimalarial activity until metabolized in the body to the active antimalarial agent cycloguanil. The usefulness of proguanil is limited by the rapid development of drug resistance by the malarial parasite. The hydrochloride is used for the casual prophylaxis of falciparum malaria, to suppress other forms of malaria, and to reduce transmission of infection (From Martindale, The Extra Pharmacopoeia, 30th ed, p405)