This study aims to demonstrate the superiority of budesonide compared to placebo as maintenance therapy in keeping patients in remission over a one-year period

Inclusion Criteria:

1. Signed informed consent,

2. Patients aged >= 18 years,

3. Histologically established diagnosis of collagenous colitis (CC) defined as:

   1. Thickened sub-epithelial collagen layer >= 10 µm on well-orientated sections,
   2. Increased amount of inflammatory cells indicating chronic inflammation in the lamina propria,

4. History of non-bloody, watery diarrhoea for more than 2 weeks prior screening in patients with newly diagnosed collagenous colitis, or history of clinical relapse for more than 1 week prior screening in patients with previously established collagenous colitis,

5. A mean of >= 3 stools/day, thereof a mean of >= 1 watery stools/day, during the week prior baseline,

6. Women of child-bearing potential and being heterosexually active have to apply appropriate contraceptive methods, e.g., hormonal contraception, intrauterine device (IUD), double-barrier method of contraception (e.g., use of a condom and spermicide), or partner has undergone vasectomy. The investigator is responsible for determining whether the subject has adequate birth control for study participation.

Exclusion Criteria:

1. Other significant abnormalities at colonoscopy that may have been the cause of diarrhoea, with the exception of colonic diverticulosis and polyps < 2 cm,
2. Infectious cause of diarrhoea,
3. Untreated active celiac disease,
4. Clinical suspicion of drug-induced collagenous colitis,
5. Any severe concomitant cardiovascular, renal, endocrine, or psychiatric disorder,
6. Abnormal hepatic function (ALT or ALP > 2.5 x upper limit of normal [ULN]), liver cirrhosis, or portal hypertension,
7. Local intestinal infection,
8. Radiation therapy towards the abdominal or pelvic region,
9. Diabetes mellitus, infection, glaucoma, tuberculosis, peptic ulcer disease, or hypertension if careful medical monitoring is not ensured,
10. Known established cataract,
11. Known hereditary problems of galactose or fructose intolerance, glucose-galactose malabsorption, sucrase-isomaltase insufficiency, Lapp lactase deficiency, or congenital lactase deficiency,
12. Established osteoporosis with T-score < -2.5,
13. Pregnancy or lactation,
14. History of cancer in the last five years,
15. History of significant bowel resection,
16. Therapy with immunomodulators (azathioprine, 6-mercaptopurine, or methotrexate) in the last 3 months,
17. Treatment with oral, rectal, or intravenous corticosteroids including budesonide in the last month,
18. Known intolerance/hypersensitivity to study drug or drugs of similar chemical structure or pharmacological profile,
19. Patients who are unable to adhere to the study visit schedule and other protocol requirements according to the judgement of the investigator,
20. Participation in another clinical trial within the last 30 days, simultaneous participation in another clinical trial, or previous participation in this trial.