Gastrointestinal symptoms, particularly diarrhoea, are a relatively common side effect of antibiotic usage. In the vast majority of cases the mechanisms involved in these side effects are not understood. Proposed mechanisms include the direct action of antibiotics on intestinal function, inducing predisposition to infection with an enteric pathogen, and factors secondary to disturbance of the normal intestinal flora that do not involve infection with a known pathogen. There is some evidence for all three potential scenarios.