Complicated intra-abdominal infections require source control when possible, e.g. a surgical procedure to remove an infection, and antibiotic therapy to obtain cure. Source control is not always possible. CABIs are associated with mortality and prolongation of hospitalisation. After apparently effective treatment, potentially including source control procedures and antibiotics, infections can relapse. There are a number of reasons for relapse; one is that antibiotic treatment may not have been given for long enough to eradicate the bacteria from, what should be, a sterile intra-abdominal cavity. Antibiotics are given until a patient is better, but not until all bacteria are eradicated, allowing them to regrow and re-start an infection. Standard antibiotic duration is variable: some doctors provide long courses and others short. We therefore want to compare durations, to see if longer courses of antibiotics are able to help prevent these relapses, or if shorter courses are as effective but have fewer side effects. We have not identified closely related strategies which may optimise the management of CABIs.