Should i take cefdinir if i have ulcerative colitis? i was prescribed it at an urgent care center for a sinus infection. i read it could cause a deadly reaction?

A little more will be needed for a specific answer. Your gastroenterologist or primary care doc would be your best resource for a definitive answer as being aware of details of your condition, current treatment and comorbidities. Having ulcerative colitis is not a contraindication for cefdinir, all drugs can cause deadly reactions if not taken properly.