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.