On the implications of endogenous resistance to medications.

Uniform treatment guidelines are often used in medicine to ensure that all physicians prescribe a safe, efficacious, and cost-effective drug in treating a medical condition. The main message of this paper is that a policy of uniform treatment based on the standard cost-effectiveness criterion may be inappropriate when drug resistance is endogenous, and selection pressure imposed by the use of any single drug (antibiotic, antiviral, or antimalarial) leads sooner or later to the evolution of resistance (by bacteria, viruses, or parasites) to that drug. The paper shows that a mixed treatment policy of multiple drug use is generally desirable, and characterizes analytically the conditions under which it is optimal.