Clostridium difficile associated diarrhoea is an important cause of morbidity in patients treated with antibiotics, especially in hospital. Clinical relapse occurs after up to 30% of initially successful treatments for colitis. Preliminary reports suggest that Rifaximin, a poorly absorbed antibiotic used to treat travellers diarrhoea can prevent relapse. We plan to carry out a randomised placebo controlled trial to test the hypothesis that Rifaximin given in a reducing dose over 4 weeks after successful treatment will reduce the relapse rate.

Aims i) To examine efficacy of a follow-on course of Rifaximin given after a successful initial course of standard treatment, in the prevention of relapse in C. difficile associated diarrhoea (CDAD).

ii) To examine changes in faecal microbiota in patients given Rifaximin vs. Placebo.

Treatment 4 weeks treatment with Rifaximin or Placebo tablets. Tapering dose starting with 2 x 200mg tablets three times a day (total = 1.2g per day) for the 1st 2 weeks, reduced to 1 x 200mg tablet three times a day (total = 0.6g per day) for the 2nd 2 weeks.

Primary endpoint: The difference in % relapse between Rifaximin and placebo at 12 weeks

Inclusion criteria:

1. Men / Women aged 18 and over (We will also include those adults who lack mental capacity for whom we have a legal representative)
2. Successful treatment of clinically diagnosed CDAD using standard therapy (metronidazole or vancomycin given according to standard local hospital guidelines).

Exclusion criteria:

1. Woman of child bearing potential and not willing to use at least one highly effective contraceptive method throughout the study
2. Male with spouse/partner of child bearing potential and not willing to use condoms
3. Pregnant or breast feeding
4. Unable to swallow tablets
5. Life expectancy of <4 weeks
6. Hypersensitivity to the active substance, to any rifamycin (e.g. rifampicin or rifabutin) or to any of its excipients (Tablet core: Sodium starch glycolate type A, glycerol distearate, colloidal anhydrous, silica, talc and microcrystalline cellulose. Tablet coating: hypromellose, titanium dioxide (E171), disodium edentate, propylene glycol and red iron oxide E172)
7. >5 days post standard therapy (metronidazole or vancomycin) for clinically diagnosed CDAD
8. Taking ciclosporin