Combination therapy is now being used for many infectious diseases, such as tuberculosis, malaria, and HIV. Combination therapy offers the potential of preventing drug resistance, because organisms resistant to one of the drugs may be susceptible to the other drug; and also the potential to diminish drug therapy duration and thus side effects. These two potential benefits to some extent contradict each other: preventing resistance is best done if full courses of both drugs is used; diminishing therapy duration means using less than the full course of each drug. The optimum combination regimen is one in which sufficient amounts of both drugs are used to have high efficacy, yet the amounts are as low as possible to spare patients unnecessarily long courses of drug.