Crohn’s disease(CD) is a chronic inflammatory condition of the intestine, which can cause damage to the portion of the small intestine where vitamin B12 is absorbed and result in B12 deficiency. There is evidence that taking vitamin B12 supplementation may help to improve low levels caused by Crohn’s disease.
However, B12 deficeincy is not inevitable. In a recent study(2010), 22% of CD pateints had low B12 levels.In CD patients, prior small intestinal surgery is an independent risk factor for having a low serum vitamin B12 level.
In general, when B12 deficiency is diagnosed, it should be treated. Routine prophylaxis with B12 is not recommended by guidelines.
Mustafa Yakut et al, Serum vitamin B12 and folate status in patients with inflammatory bowel diseases European Journal of Internal Medicine Volume 21, Issue 4 , Pages 320-323, August 2010