So I will preface this by saying I don't have Crohn's (at least we don't think so anyway), but I do have inflammation in my terminal ileum.
I have never been huge into taking multivitamins because vitamins tend to upset my stomach. Since getting sick with IBD, however, I have developed some deficiencies that require supplementation:
Vitamin D: OK, I live somewhere with little sunshine so I was already deficient in this one. But it got really dire when I got IBD. My doc prescribes a 50,000 IU Vitamin D gelcap that I take once a week. On the other days, I take a regular 1,000 IU Vitamin D/ 400 mg Calcium chew. I need the calcium because I'm on long-term prednisone.
Vitamin B-12: I have struggled with B-12 deficiency for about a decade but again, it got really serious once I got IBD. My doctor prescribes injections for this.
Vitamin C: I take a high dose (1,000 mg) vitamin C pill whenever I remember (a couple times a week). I'm on a limited diet due to my flare so I don't get hardly any fruits/veg.
Potassium: I have prescription potassium pills for when I have a lot of diarrhea and get hypokalemic.
So I guess my perspective is that you should get tested for deficiencies and worry about supplementing those (usually the dose required for this exceeds what a multivitamin can provide and often requires a prescription supplement). That said, I don't think taking a multivitamin hurts anything either.
28 year old female
Issues began in July 2014
Capsule endoscopy in late March 2017 showed "suspected Crohn's Ileocolitis"
After colonoscopy on April 14, 2017, was officially diagnosed with Ulcerative Pancolitis (w/ backwash ileitis)
Current meds: Apriso 4 x daily, Prednisone 30 mg, Canasa 1000mg suppository nightly, B12 injections monthly, 50,000 IU Vit D weekly
Started Remicade July 11, 2017