I'll be on oral prednisone for 12 days for an ms relapse. is there anything i can do to control the weight gain side effect?

YES: First, recognize the need to keep low salt intake, with some supplementation of potassium. Second, fluid retention mostly "water weight", and a mild diuretic such as hydrochlorothiazide is useful. Third, if you take all your dose once daily, side effects may be lower, and med still works fine. Am i missing something? You do not list a specific ms drug! if no dmt on board, get this started.