Inclusion Criteria:

Patient group:

• Male and female patients with Crohn's disease (diagnosed by radiology, endoscopy and pathology), with anorectal involvement.
• Aged 18-65 years.
• Have had perianal symptoms for longer than 2 months
• Have a score of 5 or above in the Perianal Crohn's Disease Activity Index (PCDAI)
• Have a score of 1 or above on the 'Type of perianal disease' assessment of the PCDAI

• Subjects can be on concomitant medication. Acceptable regimens are:

  • Aminosalicylates at a dosage that has been stable for more than 4 weeks before screening;Infliximab on a stable infusion regimen;
  • Oral corticosteroids <40mg per day; Methotrexate, provided that dosage has been stable for more than 4 weeks; azathioprine or mercaptopurine at a dosage that has been stable for more than 8 weeks;
  • Antibiotics at a dosage that has been stable for 4 weeks but excluding metronidazole.
  • Cyclosporin on a stable dose for more than 4 weeks.
• If patients have setons these must have been in place for at least 4 weeks prior to screening.
• 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 birth control, one of which may include oral contraception) for the duration of the study.
• Must have provided written informed consent to participate.

Healthy volunteer group:

• They have had surgery to the anus or rectum in the past 4 weeks.
• Have a history of inflammatory bowel disease.
• Allergic to metronidazole.
• Are taking any prohibited medication.
• Not prepared to refrain from drinking alcohol during the course of the study.
• Deemed mentally incompetent.
• Considered by their physician unlikely to be able to comply with the protocol.
• Has donated blood within the past 3 months.
• Taken part in an experimental drug study in the preceding three months.

Exclusion Criteria:

Patient group:

• 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.
• Not prepared to refrain from drinking alcohol during the course of the study.
• Deemed mentally incompetent.
• Considered by their physician unlikely to be able to comply with the protocol.
• Has donated blood within the past 3 months.
• Taken part in an experimental drug study in the preceding three months.
• Has donated blood within the past 3 months.
• Taken part in an experimental drug study in the preceding three months.

Healthy volunteer group:

• They have had surgery to the anus or rectum in the past 4 weeks.
• Have a history of inflammatory bowel disease.
• Allergic to metronidazole.
• Are taking any prohibited medication.
• Not prepared to refrain from drinking alcohol during the course of the study.
• Deemed mentally incompetent.
• Considered by their physician unlikely to be able to comply with the protocol.
• Has donated blood within the past 3 months.
• Taken part in an experimental drug study in the preceding three months.