The notion that completing the course of an antibiotic reduces the emergence of bacterial resistance is not accepted anymore. From in vitro evidence, we know that prolonged exposure to antibiotic, particularly at low levels (as occurs for instance in the oropharynx of patients treated with most oral antibiotics) is a good way to breed bacteria that are resistant – mutational change and horizontal gene transfers are potentiated by the stress induced by antibiotic exposure. Antibiotic presence provides a selective advantage for resistant sub-populations that then become dominant.