What Causes Mouth Sores While On Duramine?

I have reactive arthritis and ankylosis spondylitis. From time to time I have to go on prednisone for a couple of months to settle down severe inflammation. My problem is it makes me so hungry that I eat like a demon and am gradually gaining weight which aggravates the arthritic problems. My doctor has prescribed duramine short term to help with the hunger but I find it makes my mouth very sore. Is this a common side effect and what can I do about it?