Abstract According to the dynamic relationship between antibiotic use and the incidence of multidrug-resistant bacteria, the effectiveness of antibiotics is characterized as a natural resource exploited by antibiotic use. A major question is whether antibiotic effectiveness should be considered a renewable or an exhaustible resource. This paper presents a model combining epidemiological aspects of the decrease of resistance over time with an economic approach, modeling antimicrobial resistance as a negative externality of antibiotic use. The article concludes that the relative fitness of resistant bacteria is one driving factor for an estimation of the externality. If we, for instance, assume a low degree of relative fitness of resistant bacteria it is shown that resistance decreases fastest directly after antibiotic treatment. Accordingly, cycling of nonidentical drugs improves antibiotic effectiveness because cycling lowers the frequency of use of the individual antibiotic. In this case, antibiotic effectiveness must be characterized as a renewable resource in much the same way as a stock of fish in a lake. In contrast, if we assume a high degree of relative fitness of resistant bacteria, antibiotic effectiveness must be treated as an exhaustible resource because it is declined by every use of antibiotics.