Inclusion Criteria:

• Have Crohn's disease (diagnosed by radiology, endoscopy and pathology), with anorectal involvement.
• Have had perianal symptoms for longer than 3 months
• Have a PCDAI of 5 or above at baseline
• Subjects can be on concomitant medication. Acceptable regimes are:-
• Aminosalicylates at a dosage that has been stable for more than 4 weeks before screening;
• Oral corticosteroids <40mg per day that has been stable for more than 3 weeks;
• Methotrexate given for at least 3 months that has been stable for more than 4 weeks; Azathioprine or mercaptopurine given for at least 6 months at dosage that has been stable for more than 8 weeks;
• Antibiotics at a dosage that has been stable for 4 weeks (subjects may be on oral metronidazole but on a dose not more than 750mg per day),
• Cycolsporin for more than 3 months and on a stable dose for more than 4 weeks;
• Patients who have been treated with Infliximab must have received their initial dose 3 months before starting study medication and their most recent dose at least 8 weeks before starting study medication and not receive an infusion whilst in the double-blind treatment phase of the study for 4 weeks. During the second 4 week open stage of the study patients can commence again a maintenance regime with Infliximab treatment if deemed necessary
• Any patients not on concurrent medication must have been off medication for at least 4 weeks before screening.
• If patients have Setons these must have been in place for at least 4 weeks prior to screening.
• Subjects must be aged 18 years or over and of the legal age of consent.
• If female, the subject must not be lactating and must be (a) post- menopausal, (b) surgically sterilised, or (c) have a negative pregnancy test result prior to entry into the study and will use double- barrier methods of contraception (two separate methods of birthcontrol one of which may include oral contraception) for the duration of the study.
• Must have provided written informed consent to participate.

Exclusion Criteria:

• They have had surgery to the anus or rectum in the past 4 weeks;
• They have a perianal abscess requiring incision and drainage;
• They have a stoma of less than 6 months duration;
• Allergic to metronidazole;
• Are taking any prohibited medication.
• Deemed mentally incompetent.
• Considered by their physician unlikely to be able to comply with the protocol.
• Taken part in an experimental drug study in the preceding three months.