You mention that you are doing fine on 500mg once a day of Mesalazine. This is very reassuring, and there may be a good case for you to stop the mesalazine as a trial to see if that causes a flare up of the disease. The risks of a flare up of the left sided colitis following stoppage of the Mesalazine in mild left sided colitis are small, although they are not completely absent. If you do have a flare up, it can be quickly controlled using some Mesalazine enemas, but that will also make you more convinced for the need to take Mesalazine in a dose of at least 1000 mg a day as maintenance therapy life long.