Mesacol can rarely cause 'interstitial nephritis', which means inflammation of kidneys. It can be diangosed urine studies, kidney imagaing and in some cases by kidney biopsy. When this occurs, this involves both kidneys. The fact that you have only left kidney that is affected, you need to undergo thorough evaluation to find out exact cause of your kidney disease/creatinine elevation. Please see your local nephrologist as soon as possible. If it is determined that you don't have interstitial nephritis, you can continue to take mesacol for ulcerative colitis.